Table of contents

  1. Kosmonaut in 2020
  2. WebKit in 2020
  3. My plans for 2021

I spent much of 2020 working on web browsers (mainly Kosmonaut and WebKit). Here’s a little write-up about how that was, and what I have planned for the future.

Kosmonaut in 2020

One year ago today, I was finishing the first implementation of block layout and display-list generation. In terms of LoC, Kosmonaut hasn’t grown much — from 7.5k then to 20k now. However, there are some pretty neat improvements wrapped up in that 12.5k LoC increase:

  • Layout-tree dump1 snapshot based testing
  • Partial support2 for abstract box layout with the writing-mode and direction properties
  • Support for arbitrary scale factors, e.g. for HiDPI screens
  • OpenGL-based box painting and text rendering (though text rendering is not yet hooked into layout)

Three months ago, I ventured out to rewrite Kosmonaut’s layout engine, as I had made a mess of it in my first pass at implementing abstract box layout. The result is this PR, merged today, that ended up being a rewrite of…a lot of things. Some highlights from that PR include:

  • Proper representation of many spec-level concepts, such as text runs, formatting contexts, various types of boxes, and more.
  • Much cleaner layout tree dump output. Before vs. after
  • A much cleaner HiDPI scaling implementation. The TLDR of this is that scaling used to be done in layout, which required passing a scale_factor everywhere and applying it in exactly the right places. The new implementation scales the viewport size down before layout and scales everything else up just before painting, making things far more simple and less prone to bugs.

There’s a fair amount of cleanup I’d like to do following the landing of this PR, but it’s a big step forward and I’m really happy with how it turned out.

One of the reasons this took so long is because I spent quite a long time trying out various designs and subsequently throwing them out. One thing I really wanted to do with this PR was to enforce certain spec-level constraints with the type system. To give a concrete example, take this snippet from the CSS Display spec regarding block containers:

A block container either contains only inline-level boxes participating in an inline formatting context, or contains only block-level boxes participating in a block formatting context.

I had experimented with a design that looked like this:

pub struct BlockContainer {}
pub struct BlockLevelBlockContainer {
   children: Vec<BlockLevelBox>
}
pub struct InlineLevelBlockContainer {
   children: Vec<InlineLevelBox>
}

impl BlockContainer {
   pub fn new(...) -> BlockContainer {}

   pub fn add_block_level_child(
      blb: BlockLevelBox
   ) -> BlockLevelBlockContainer {}

   pub fn add_inline_level_child(
      ilb: InlineLevelBox
   ) -> InlineLevelBlockContainer {}
}

The idea is that you could only get a BlockLevelBlockContainer or InlineLevelBlockContainer by calling one of these add_{block, inline}_level_child methods (i.e. neither of these structs have new methods), ensuring at compile-time that the quoted spec invariant is upheld.

However, this proved to to be awkward for a number of reasons, and I scrapped the idea in favor of a less restrictive API that technically allows these rules to be broken. I would like to revisit this someday, as allowing users to encode rules via the type system (and making it ergonomic to do so) is something Rust is good at.

WebKit in 2020

Kosmonaut was not the only browser I worked on in 2020 — this year also saw the beginning of my contributions to WebKit. I landed 13 patches, and worked on quite a few others I didn’t push across the finish line. Here are some highlights:

  • Various bug fixes and improvements to WebKit’s CSS variables implementation. I have a blog post in the works describing these in more detail, so stay tuned.
  • Fixed a bug that caused ::selection pseudoelement styles to not be applied to elements with direct anonymous parents, such as text nodes. [commit]
  • Per spec, ignore order when parsing <inset> and <color> values for the box-shadow property. This makes values that should’ve been valid for box-shadow actually work. [commit]

In the second half of the year I really felt like I was getting into a groove in working on WebKit, so I’m excited to get back to it.

My plans for 2021

2021 has arrived! Here are some things I’d like to complete in Kosmonaut soon:

  • Parse and expand the most common CSS shorthands — margin, border, padding, etc. Kosmonaut currently only understands longhands, which is quickly becoming inconvenient.
  • Add the ability to load styles from <link href="..."> and <style></style> tags. Currently, loading HTML and CSS in Kosmonaut requires passing all files individually via the —files flag, which is inconvenient (are you noticing a trend?).
  • Handle display: none boxes
  • Further improve support for abstract box layout. Concretely, this means making writing-mode: {sideways-lr, sideways-rl, vertical-rl} and permutations with direction: {ltr, rtl} work. I don’t think this will be all that hard. Calculation of block start coordinates and inline start coordinates will have to change, and maybe a few other things.
  • A basic implementation of inline layout, followed by some polish of Kosmonaut’s existing OpenGL text rendering.

After this list, I’m not really sure what will come next for Kosmonaut. I think it would be a fun minigame to start tackling the Acid2 test and/or some web platform tests.

I’ve taken a break from WebKit recently to push Kosmonaut’s layout rewrite across the finish line, but I want to get back to it soon. I have been pondering about an experiment I’d like to try regarding my work in WebKit, so stay tuned for information on that — I’ll be posting about it here.


  1. Here’s what a layout dump looks like.  

  2. The specifics of what “partial support” means is documented here