Lightpanda migrate DOM implementation to Zig
lightpanda.io190 points by gearnode a day ago
190 points by gearnode a day ago
This reminds me of the Servo project's journey. Always impressed to see another implementation of the WHATWG specs.
It's interesting to see Zig being chosen here over Rust for a browser engine component. Rust has kind of become the default answer for "safe browser components" (e.g., Servo, Firefox's oxidation), primarily because the borrow checker maps so well to the ownership model of a DOM tree in theory. But in practice, DOM nodes often need shared mutable state (parent pointers, child pointers, event listeners), which forces you into Rc<RefCell<T>> hell in Rust.
Zig's manual memory management might actually be more ergonomic for a DOM implementation specifically because you can model the graph relationships more directly without fighting the compiler, provided you have a robust strategy for the arena allocation. Excited to learn from Lightpanda's implementation when it's out.
Hi, I am Francis, founder of Lightpanda. We wrote a full article explaining why we choose Zig over Rust or C++, if you are interested: https://lightpanda.io/blog/posts/why-we-built-lightpanda-in-...
Our goal is to build a headless browser, rather than a general purpose browser like Servo or Chrome. It's already available if you would like to try it: https://lightpanda.io/docs/open-source/installation
I see you're using html5ever for HTML parsing, and like it's trait/callback based API (me too). It looks like style/layout is not in scope at the moment, but if you're ever looking at adding style/layout capabilities to lightpanda, then you may find it useful to know that Stylo [0] (CSS / style system) and Taffy [1] (box-level layout) are both avaiable with a similar style of API (also Parley [2] which has a slightly different API style but can be combined with Taffy to implement inline/text layout).
[0]: https://github.com/servo/stylo
[1]: https://github.com/DioxusLabs/taffy
[2]: https://github.com/linebender/parley
---
Also, if you're interested in contributing C bindings for html5ever upstream then let me know / maybe open a github issue.
Off topic note: I read the website and a few pages of the docs and it's unclear to me for what I can use LightPanda safely. Like say I wanted to swap my it as my engine on playwright, what are the tradeoffs? What things are implemented, what isnt?
Thanks for the feedback, we will try to make this clearer on the website. Lightpanda works with Playwright, and we have some docs[1] and examples[2] available.
Web APIs and CDP specifications are huge, so this is still a work in progess. Many websites and scripts already work, while others do not, it really depends on the case. For example, on the CDP side, we are currently working on adding an Accessibility tree implentation.
[1] https://lightpanda.io/docs/quickstart/build-your-first-extra...
[2] https://github.com/lightpanda-io/demo/tree/main/playwright
Maybe you should recommend a recipe for configuring playwright with both chromium and lightpanda backends so a given project can compare and evaluate whether lightpanda could work given their existing test cases.
I was actually interested into using lightpanda for E2Es to be honest, because halving the feedback cycle would be very valuable to me.
I think it's really more of an alternative to JSDom than it is an alternative to Chromium. It's not going to fool any websites that care about bots into thinking it's a real browser in other words.
Would be helpful to compare Lightpanda to Webkit, Playwright has a driver for example and its far faster and less resource hungry than Chrome.
When I read your site copy it struck me as either naive to that, or a somewhat misleading comparison, my feedback would be just to address it directly alongside Chrome.
Thanks Francis, appreciate the nice & honest write-up with the thought process (while keeping it brief).
Would be great if it could be used as a wasm library... Just saying... Is it? I would actually need and use this.
Choosing something like Zig over C++ on simplicity grounds is going to be a false economy. C++ features exist for a reason. The complexity is in the domain. You can't make a project simpler by using a simplistic language: the complexity asserts itself somehow, somewhere, and if a language can't express the concept you want, you'll end up with circumlocution "patterns" instead.
Build system complexity disappears when you set it up too. Meson and such can be as terse as your Curl example.
I mean, it's your project, so whatever. Do what you want. But choosing Zig for the stated reasons is like choosing a car for the shape of the cupholders.
Your Swiss Army Knife with a myriad of 97 oddly-shaped tools may be able to do any job anyone could ask of it, but my Swiss Army Knife of 10 well-designed tools that are optimal for my set of tasks will get my job done with much less frustration.
> C++ features exist for a reason.
But sometimes not good ones. Lot's of domains make tradeoffs about what features of C++ to actually make use of. It's an old language with a lot of cruft being used across a wide set of problems that don't necessarily share engineering trade offs.
That’s not fully true though. There’s different types of complexity:
- project requirements
- requirements forced upon you due to how the business is structured
- libraries available for a particular language ecosystem
- paradigms / abstractions that a language is optimised for
- team experiences
Your argument is more akin to saying “all general purpose languages are equal” which I’m sure you’d agree is false. And likewise, complexity can and will manifest itself differently depending on language, problems being solved, and developer preferences for different styles of software development.
So yes, C++ complexity exists for a reason (though I’d personally argue that “reason” was due to “design by committee”). But that doesn’t mean that reason is directly applicable to the problems the LightPanda team are concerned about solving.
C++ features for complexity management are not ergonomic though, with multiple conflicting ideas from different eras competing with each other. Sometimes demolition and rebuild from foundations is paradoxically simpler.
A lot of them only still exist for backwards compatabilities sake though. And a decent amount because adding something as a language extension rather than building the language around it has consequences.
C++ features exist for a reason but it may not be a reason that is applicable to their use case. For example, C++ has a lot of features/complexity that are there primarily to support low-level I/O intensive code even though almost no one writes I/O intensive code.
I don't see why C++ would be materially better than Zig for this particular use case.
Respectfully, for browser-based work, simplicity is absolutely not a good enough reason to use a memory-unsafe language. Your claim that Zig is in some way safer than Rust for something like this is flat out untrue.
What is your attack model here? Each request lives in its own arena allocator, so there is no way for any potentially malicious JavaScript to escape and read memory owned by any other request, even if there is a miscode. otherwise, VM safety is delegated to the V8 core.
I don't think that a language that was meant to compete with C++ and in 10+ years hasn't captured 10% of C++'s (already diminished) market share could be said to have become "kind of the default" for anything (and certainly not when that requires generalising from n≅1).
It has for Amazon, Adobe, Microsoft, Google and the Linux kernel.
It remains to be seen which big name will make Zig unavoidable.
> It has for Amazon, Adobe, Microsoft, Google and the Linux kernel.
I don't think so. I don't know about Adobe, but it's not a meaningful statement for the rest. Those companies default to writing safe code in languages other than Rust, and the Linux kernel defaults to unsafe code in C. BTW, languages favoured by those projects/companies do not reliably represent industry-wide preferences, let alone defaults. You could certainly say that of the two languages accepted so far in the Linux kernel, the only safe one is Rust, but there's hardly any "default" there.
> It remains to be seen which big name will make Zig unavoidable.
I have no idea whether or not Zig will ever be successful, but at this point it's pretty clear that Rust's success has been less than modest at best.
It is a clear mandate on those companies that whatever used to be C or C++, should be written in Rust for green field development.
Whatever could be done in programming languages with automatic memory management was already being done.
Anyone deploying serverless code into Amazon instances is running of top of Firecracker, my phone has Rust code running on it, and whatever Windows 11 draws something into the screen, it goes through Rust rewrite of the GDI regions logic, all the Azure networking traffic going through Azure Boost cards does so via Rust firmware.
Adobe is the sponsor for the Hylo programming language, and key figures in the C++ community, are doing Rust talks nowadays.
"Adobe’s memory safety roadmap: Securing creativity by design"
https://blog.adobe.com/security/adobes-memory-safety-roadmap...
Any hobby language author would like to have 1% of the said modest Rust's success, I really don't get the continuous downplay of such achievement.
> It is a clear mandate on those companies that whatever used to be C or C++, should be written in Rust for green field development. Whatever could be done in programming languages with automatic memory management was already being done.
I don't know how true either of these statements is or to what extent the mandate is enforced (at my company we also have language mandates, but what they mean is that to use a different language all you need is an explanation and a manager to sign off), but I'll ask acquaintances in those companies (Except Adobe; don't know anyone there. Although the link you provided doesn't say Rust; it says "Rust or Swift". It also commits only to "exploring ways to reduce the use of new C and C++ code in safety critical parts of our products to a fraction of current levels").
What I do know is that the rate at which Rust is adopted, is significantly lower than the rate at which C++, Java, C#, Python, TS, and even Go were adopted, even in those companies.
Now, there's no doubt that Rust has some real adoption, and much more than just hobby languages. Its rate of adoption is significantly higher than that of Haskell, or Clojure, or Elixir were (but lower than that of Ruby or PHP). That is without a doubt a great accomplishment, but not what you'd expect from a language that wishes to become the successor to C++ (and doesn't suffer from lack of hype despite its advanced age). Languages that offer a significant competitive advantage, or even the perception of one, spread at a faster pace, certainly those that eventually end up in the top 5.
I also think there's little doubt that the Rust "base" is more enthusiastic than that of any language I remember except maybe that of Haskell's resurgence some years back (and maybe Ruby), and that enthusiasm may make up for what they lack in numbers, but at some point you need the numbers. A middle-aged language can only claim to be the insurgent for so long.
>>It is a clear mandate on those companies that whatever used to be C or C++, should be written in Rust for green field development. >>Any hobby language author would like to have 1% of the said modest Rust's success, I really don't get the continuous downplay of such achievement.
This is a political achievement, not technical one. People are bitter about it as it doesn't feel organic and feel pushed onto them.
There is technical achievement in:
> Anyone deploying serverless code into Amazon instances is running of top of Firecracker, my phone has Rust code running on it, and whatever Windows 11 draws something into the screen, it goes through Rust rewrite of the GDI regions logic, all the Azure networking traffic going through Azure Boost cards does so via Rust firmware.
Ignoring it doesn't make those achievements political rather than technical.
While certain teams within Google are using rust by default, I'm not sure rust is anywhere close in scale for new lines of code committed per week to c++.
For Android specifically, by Q3 of last year more new lines of Rust were being added per week than new lines of C++: https://security.googleblog.com/2025/11/rust-in-android-move...
The problem is that the number of browser engines is n=2.
Interestingly, Ladybird, which aims at being the n = 3, is also written in C++.
And use-after-free, when that arena's memory goes away.
But arenas have substantial benefits. They may be one of the few remaining reasons to use a low-level (or "systems programming") language in the first place. Most things are tradeoffs, and the question isn't what you're giving up, but whether you're getting the most for what you're paying.
Arenas are also available in languages with automatic memory management, e.g. D, C# and Swift, to use only modern languages as example.
Thus I don't consider that a reason good enough for using Zig, while throwing away the safety from modern languages.
First, Zig is more modern than any of the languages you mention. Second, I'm not aware that any of those languages offer arenas similar in their power and utility to Zig's while offering UAF-freedom at the same time. Note that "type-safe" arenas are neither as powerful as general purpose arenas nor fully offer UAF-freedom. I could be wrong (and if I am, I'd really love to see an arena that's both general and safe), but I believe that in all these languages you must compromise on either safety or the power of the arena (or both).
> First, Zig is more modern than any of the languages you mention
How so? This feels like an empty statement at best.
"modern: relating to the present or recent times as opposed to the remote past". I agree it's not a useful concept here but I didn't bring it up. Specifically, I don't think there's any consideration that had gone into the design of D, C#, or Rust that escaped Zig's designer. He just consciously made different choices based on the data available and his own judgment.
Not really modern, it is Object Pascal/Modula-2 repackaged in C like syntax.
The only thing relatively modern would be compile time execution, if we forget about how long some languages have had reader macros, or similar capabilities like D's compile time metaprogramming.
Also it is the wrong direction when the whole industry is moving into integrity by default on cyber security legislation.
There are several examples around of doing arenas in said languages.
https://dlang.org/phobos/std_experimental_allocator.html
You can write your own approach with the low level primitives from Swift, or ping back into the trusty NSAutoreleasePool.
One example for C#, https://github.com/Enichan/Arenas
> Not really modern, it is Object Pascal/Modula-2 repackaged in C like syntax.
That's your opinion, but I couldn't disagree more. It places partial evaluation as its biggest focus more so than any other language in history, and is also extraordinarily focused on tooling. There isn't any piece of information nor any technique that was known to the designers of those older languages and wasn't known to Zig's designer. In some situations, he intentionally chose different tradeoffs on which there is no consensus. It's strange to insist that there is some consensus when many disagree.
I have been doing low-level programming (in C, C++, and Ada in the 90s) for almost 30 years, and over that time I have not seen a low-level language that's as revolutionary in its approach to low-level programming as Zig. I don't know if it's good, but I find its design revolutionary. You certainly don't have to agree with my assessment, but you do need to acknowledge that some people very much see it that way, and don't think it's merely a "repackaged" Pascal-family language in any way.
I guess you could say that you personally don't care about Zig's primary design points and when you ignore them you're left with something that you find similar to other languages, but that's like saying that if you don't care about Rust's borrow- and lifetime checking, it's basically just a mix of C++ and ML. It's perfectly valid to not care about what matters most to some language's designer, and it's perfectly valid to claim that what matters to them most is misguided, but it is not valid to ignore a language's design core when describing it just because you don't care about it.
> Also it is the wrong direction when the whole industry is moving into integrity by default on cyber security legislation.
Again, that is an opinion, but not one I agree with. For one, Rust isn't as safe as other safe languages given its relatively common reliance on unsafe. If spatial and temporal memory safety were the dominating concerns, there wouldn't be a need for Rust, either (and it wouldn't have exposed unsafe). Clearly, everyone recognises that there are other concerns that sometimes dominate, and it's pretty clear that some people, who are no less knowledgeable about the software industry and its direction, prefer Zig. There is no consensus here either way, and I'm not sure there can be one. They are different languages that suit different people/projects' preferences.
Now, I agree that there's definitely more motion toward more correctness - which is great! - and I probably wouldn't write a banking or healthcare system in Zig, but I wouldn't write it in Rust, either. People reach for low level languages precisely when there may be a need to compromise on safety in some way, and Rust and Zig make different compromises, both of which - as far as I can tell - can be reasonable.
> There are several examples around of doing arenas in said languages.
From what I can tell, all of them either don't provide freedom from UAF, or they're not nearly as general as a proper arena.
I know of one safe and general arena design in RTSJ, which immidiately prevents a reference to a non-enclosing arena from being written into an object, but it comes with a runtime cost (which makes sense for hard realtime, where you want to sacrifice performance for worst-case predictability).
> You certainly don't have to agree with my assessment, but you do need to acknowledge that some people very much see it that way, and don't think it's merely a "repackaged" Pascal-family language in any way.
My opinion is that 99% of those people never knew anything beyond C and C++ for systems programming, and even believe the urban myth that before C there were no systems programming languages.
Similar to those that only discover compiled languages and type systems exist, after spending several years with Python and JavaScript, and then even Go seems out of this world.
Wouldn't C# and Swift make it tough to integrate with other languages? Whereas something written in Zig (or Rust) can integrate with anything that can use the C ABI?
Yeah that's certainly possible but leaking a pointer like this seems like it would be really easy to spot?
It's harder than you'd expect. Depending on what kind of bucketing an arena does (by size or by type), a stale reference may end up pointing to another piece of memory of the correct type, which is still wrong, but more subtly than a crash.
Too late now, but is the requirement for shared mutable state inherent in the problem space? Or is it just because we still thought OOP was cool when we started on the DOM design?
Yes. It is required for W3C's DOM APIs, which give access to parent nodes and allow all kinds of mutations whenever you want.
Event handlers + closures also create potentially complex situations you can't control, and you'll need a cycle-breaking GC to avoid leaking like IE6 did.
You can make a more restricted tree if you design your own APIs with immutability/ownership/locking, but that won't work for existing JS codebases.
> without fighting the compiler
It's unfortunate that "writing safe code" is constantly being phrased in this way.
The borrow checker is a deterministic safety net. Claiming Zig is easier ignores that its lack of safety checks is what makes it feel easier; if Zig had Rust’s guarantees, the complexity would be the same. Comparing them like this is apples vs. oranges.
That's a very narrow way of looking at things. ATS has a much stronger "deterministic safety net" than Rust, yet the reason to use Rust over ATS is that "fighting the compiler" is easier in Rust than in ATS. On the other hand, if any cost is worth whatever level of safety Rust offers for any project, than Rust wouldn't exist because there are far more popular languages with equal (or better) safety. So Rust's design itself is an admission that 1. more compile-time safety is always better, even if it complicates the language (or everyone who uses Rust should use ATS), and 2. any cost is worth paying for safety (or Rust wouldn't exist in the first place).
Safety has some value that isn't infinite, and a cost that isn't zero. There are also different kinds of safety with different value and different costs. For example, spatial memory safety appears to have more value than temporal safety (https://cwe.mitre.org/top25/archive/2025/2025_cwe_top25.html) and Zig offers spatial safety. The question is always what you're paying and what you're getting in return. There doesn't appear to be a universal right answer. For some projects it may be worth it to pay for more safety, and for other it may be better to pay for something else.
Imo "safety" in safe Rust is higher than it is in more popular languages.
Data races, type state pattern, lack of nulls, ...
This is comparing what Rust has and other languages don't without also doing the opposite. For example, Java doesn't enforce data-race freedom, but its data races are safe, which means you can write algorithms with benign races safely (which are very useful in concurrent programming [1]), while in Rust that requires unsafe. Rust's protection against memory leaks that can cause a panic is also weaker, as is Rust's ability to recover from panics in general. Java is now in the process of eliminating the unsafe escape hatch altogether except for FFI. Rust is nowhere near that. I.e. sometimes safe Rust has guarantees that mean that programs need to rely on unsafe code more so than in other languages, which allows saying that safe Rust is "safer" while it also means that fewer programs are actually written purely in safe Rust. The real challenge is increasing safety without also increasing the number of programs that need to circumvent it or increasing the complexity of the language further.
[1]: A benging race is when multiple tasks/threads can concurrently write to the same address, but you know they will all write the same value.
You’re changing the argument. The point wasn’t whether more safety is “worth it”, but that comparing ease while ignoring which invariants are enforced is misleading. Zig can feel simpler because it encodes fewer guarantees. I’m not saying one approach is better, only that this comparison shifts the goalposts.
Then we're in agreement. Both languages give you something that may be important, but it has a price.
> 1. more compile-time safety is always better, even if it complicates the language (or everyone who uses Rust should use ATS), and 2. any cost is worth paying for safety (or Rust wouldn't exist in the first place).
You keep repeating this. It's not true. If what you said was true, Rust would have adopted HKT, and God knows whatever type astronomy Haskell & Scala cooked up.
There is a balancing act, and Rust decided to plant a flag in memory safety without GC. The fact that Zig, didn't expand on this, but went backwards is more of an indictment of programmers unwilling to adapt and perfect what came before, but to reinvent it in their own worse way.
> There are also different kinds of safety with different value and different costs. For example, spatial memory safety appears to have more value than temporal safety (https://cwe.mitre.org/top25/archive/2025/2025_cwe_top25.html)
How did you derive this from the top 25 of CWEs? Let's say you completely remove the spatial memory issues. You still get temporal memory issues at #6.
Rust does have a GC, but I agree it planted its flag at some intermediate point on the spectrum. Zig didn't "go backwards" but planted its own flag ever so slightly closer to C than to ATS (although both Rust and Zig are almost indistinguishable from C when compare to ATS). I don't know if where Rust planted its flag is universally better than where Zig planted its flag, but 1. no one else does either, 2. both are compromises, and 3. it's uncertain whether a universal sweet spot exists in the first place.
> How did you derive this from the top 25 of CWEs? Let's say you completely remove the spatial memory issues. You still get temporal memory issues at #6.
Sure, but spatial safety is higher. So if Rust's compromise, we'll exact a price on temporal safety and have both temporal and spatial safety, is reasonable, then so is Zig's that says, the price on temporal safety is too high for what you get in return, but spatial safety only is a better deal. Neither go as far as ATS in offering, in principle, the ability to avoid all bugs. Nobody knows whether Rust's compormise is universally better than Zig's or vice versa (or perhaps neither is universally better), but I find it really strange to arbitrarily claim that one compromise is reasonable and the other isn't, where both are obviously compromises that recognise there are different benefits and different costs, and that not every benefit is worth any cost.
> Rust does have a GC
It doesn't. Not by any reasonable definition of having a GC.
And "opt-in non-tracing GC that isn't used largely throughout the standard library" is not a reasonable definition.
> Nobody knows whether Rust's compormise is universally better than Zig's
When it comes to having more segfaults, we know. Zig "wins" most segfaults per issue Razzie Award.
This is what happens when you ignore one type of memory safety. You have to have both. Just ask Go.
> And "opt-in non-tracing GC that isn't used largely throughout the standard library" is not a reasonable definition.
Given that refcounting and tracing are the two classic GC algorithms, I don't see what specifying "non tracing" here does, and reference-counting with special-casing of the one reference case is still reference counting. I don't know if the "reasonable definition" of GC matters at all, but if it does, this does count as one.
I agree that the one-reference case is handled in the language and the shared reference case is handled in the standard library, and I think it can be reasonable to call using just the one-reference case "not a GC", but most Rust programs do use the GC for shared references. It is also true that Rust depends less on GC than Java or Go, but that's not the same as not having one.
> When it comes to having more segfaults, we know. Zig "wins" most segfaults per issue Razzie Award.
And Rust wins the Razzie Award for most painful development and lack of similarly powerful arenas. It's like declaring that you win by paying $100 for something while I paid $50 for something else without comparing what we got for the money, or declaring that you win by getting a faster car without looking at how much I paid for mine.
> This is what happens when you ignore one type of memory safety.
When you have less safety for any property, you're guarnateed to have more violations. This is what you buy. Obviously, this doesn't mean that avoiding those extra violations is necessarily worth the cost you pay for that extra safety. When you buy something, looking just at what you pay or just at what you get doesn't make any sense. The question is whether this is the best deal for your case.
Nobody knows if there is a universal best deal here let alone what it is. What is clear is that nothing here is free, and that nothing here has infinite value.
> I don't know if the "reasonable definition" of GC matters at all
If you define all non-red colors to be green, it is impossible to talk about color theory.
> And Rust wins the Razzie Award for most painful development and lack of similarly powerful arenas.
That's a non-quantifiable skill issue. Segfaults per issue is a quantifiable thing.
> When you have less safety for any property, you're guarnateed to have more violations.
If that's what you truly believed outside some debate point. Then you'd be advocating for ATS or Ada.SPARK, not Zig.
> If you define all non-red colors to be green, it is impossible to talk about color theory.
Except reference counting is one of the two classical GC algorithms (alongside tracing), so I think it's strange to treat it as "not a GC". But it is true that GC/no-GC distinction is not very meaningful given how different the tradeoffs that different GC algorithms make are. Even within these basic algorithms there are combinations. For example a mark-and-sweep collector is quite different from a moving collector, or CPython uses refcouting for some things and tracing for others.
> That's a non-quantifiable skill issue. Segfaults per issue is a quantifiable thing.
That it's not as easily quantifiable doesn't make it any less real. If we compare languages only by easily quantifiable measures, there would be few differences between them (and many if not most would argue that we're missing the differences that matter to them most). For example, it would be hard to distinguish between Java and Haskell. It's also not necessarily a "skill issue". I think that even skilled Rust users would admit that writing and maintaining a large program in TypeScript or Java takes less effort than doing the same in Rust.
Also, ATS has many more compile-time safety capabilities than either Rust or Zig (in fact, compared to ATS, Rust and Zig are barely distinguishable in what they can guarantee at runtime), so according to your measure, both Rust and Zig lose when we consider other alternatives.
> Then you'd be advocating for ATS or Ada.SPARK, not Zig.
Quite the opposite. I'm pointing out that, at least as far as this discussion goes, every added value comes with added cost that needs to be considered. If what you truly believed is that more compile-time safety always wins, then it is you who should be advocating for ATS over Rust. I'm saying that we don't know where the cost-benfit sweet point is or, indeed, even if there's only one such sweey point or multiple. I'm certainly not advocating for Zig as a universal choice. I'm advocating for selecting the right tradeoffs for every project, and I'm rejecting the claim that whatever benefits Rust or Zig have compared to the other are free. Both (indeed, all languages) require you to pay in some way to get what they're offering. In other words, I'm advocating can both be more or less appropriate than the other, depending on the situation and against the position that Rust is always superior, which is based on only looking at its advantages and ignoring its disadvantages (which, I think, are quite significant).