Posted by embedding-shape 1/27/2026
Here's my own screenshot of it rendering my blog - https://bsky.app/profile/simonwillison.net/post/3mdg2oo6bms2... - it handles the layout and CSS gradiants really well, renders the SVG feed icon but fails to render a PNG image.
I thought "build a browser that renders HTML+CSS" was the perfect task for demonstrating a massively parallel agent setup because it couldn't be productively achieved in a few thousand lines of code by a single coding agent. Turns out I was wrong!
I have one project Claude is working on right now where I'm testing a setup to attempt to take myself more out of the loop, because that is the hard part. It's "easy" to get an agent to multiply your output. It's hard to make that scale with your willingness to spend on tokens rather than with your ability to read and review and direct.
I've ended up with roughly this (it's nothing particularly special):
- Runs a evaluator that evaluates the current state and assigns scores across multiple metrics.
- If a given score is above a given threshold, expand the test suite automatically.
- If the score is below a given threshold, spawn a "research agent" that investgates why the scores don't meet expectations.
- The research agent delivers a report, that is passed to an implementation agent.
- The main agent re-runs the scoring, and if it doesn't show an improvement on one or more of the metrics, the commit is discarded, and notes made of what was tried, and why it failed.
It takes a bit of trial and error to get it right (e.g. "it's the test suite that is wrong" came up early, and the main agent was almost talked into revising the test suite to remove the "problematic" tests) but a division sort of like this lets Claude do more sensible stuff for me. Throwing away commits feels drastic - an option is to let it run a little cycle of commit -> evaluate -> redo a few times before the final judgement, maybe - but it so far it feels like it'll scale better. Less crap makes it into the project.
And I think this will work better than to treat these agents as if they are developers whose output costs 100x as much.
Code so cheap it is disposable should change the workflows.
So while I agree this is a better demonstration of a good way to build a browser, it's a less interesting demonstration as well. Now that we've seen people show that something like FastRender is possible, expect people to experiment with similarly ambitious projects but with more thought put into scoring/evaluation, including on code size and dependencies.
Just the day(s) before, I was thinking about this too, and I think what will make the biggest difference is humans who posses "Good Taste". I wrote a bunch about it here: https://emsh.cat/good-taste/
I think the ending is most apt, and where I think we're going wrong right now:
> I feel like we're building the wrong things. The whole vibe right now is "replace the human part" instead of "make better tools for the human part". I don't want a machine that replaces my taste, I want tools that help me use my taste better; see the cut faster, compare directions, compare architectural choices, find where I've missed things, catch when we're going into generics, and help me make sharper intentional choices.
But for other projects, being able to scale with little or no human involvement suddenly turns some things that were borderline profitable or not possible to make profitable at all with current salaries vs. token costs into viable businesses.
Where it works, it's a paradigm shift - for both good and bad.
So it depends what you're trying to solve for. I have projects in both categories.
It's great to see hackernews be so core part of it haha.
> I thought "build a browser that renders HTML+CSS" was the perfect task for demonstrating a massively parallel agent setup because it couldn't be productively achieved in a few thousand lines of code by a single coding agent. Turns out I was wrong!
I do wonder if tech people from future/present are gonna witness this as a goliath vs david story. 20k 1 human 1 agent beats 5 million$ 1.6 millions loc browser changing how even the massive AI users/pioneers at the time thought about the use of AI
Looks like I have watched some documentaries recently but why do I feel like a documentary about this whole thing can be created in future.
But also, More and more I am feeling like AI is an absolute black box, nobody knows how to do things but we are all kind of doing experiments with it and seeing what sticks (like how we now have definitive proof that 1 human 1 agent > many agents no human in the loop)
And this is when we are 1 month in 2026, who knows what other experiments and proofs happen this year to find more about this black box, and about its usefulness or not.
Simon, it would be interesting if you could read the thread of predictions of 2026 thread in hn each month or quaterly to see how many people were wrong or right about AI as we figure out more things perhaps.
But we're very far from a browser here, so that's not that impressive. Writing a basic renderer is really not that hard, and matches the effort and low LoC from that experiment. This is similar to countless graphical toolkits that have been written since the 70s.
I know Servo has a "no AI contribution" policy, but I still would be more impressed by a Servo fork that gets missing APIs implemented by an AI, with WPT tests passing etc. It's a lot less marketable I guess. Go add something like WebTransport for instance, it's a recent API so the spec should be properly written and there's a good test suite.
The fact that it compiles is better the the cursor dude. "It Compiles" is a very low bar to working software.
Unfortunately, this context is kind of implicit, I don't actually mention it in the blog post, which I probably should have done, that's my fault.
> I get to evaluate on stuff like links being consistently blue and underlined
Yeah, this browser doesn't have a "default stylesheet" like a regular browser. Probably should have added that, but was mostly just curious about rendering the websites from the web, rather than using what browsers think the web should look like.
> It may be that some of the rendering is not supported on windows- the back button certainly isn't.
Hmm, on Windows 11 the back button should definitively work, tried that just last night. Are you perhaps on Windows 10? I have not tried that myself, should work but might be why.
After three days, I have it working with around 20K LOC, whereas ~14K is the browser engine itself + X11, then 6K is just Windows+macOS support.
Source code + CI built binaries are available here if you wanna try it out: https://github.com/embedding-shapes/one-agent-one-browser
it's amazing how far we've come in 20 years. i was a (very minor) contributor to khtml/konqueror (before apple got involved w/ webkit) in the early 2000s, and back then it was such a labor intensive process to even create a halfway working engine. like, months of work just to get basic rendering somewhat correct on a very small portion of the web (which was obv much smaller)
in addition to agentic coding, i think for this specific task having css-spec/html-spec/web-platform-tests as machine readable test suites helps a LOT. the agent can actually validate against real specs.
back in the day, despite having gecko as an open source reference, in practice the "standards" were whatever IE was doing. so you'd spend weeks implementing something only to discover every site was coded for IE's quirks lmao. for all of their other faults, google/apple and other contributors helped bring in discipline to that.
You know, I placed the specs in the repository with that goal (even sneaked in a repo that needs compiling before being usable), but as far as I can see, the agent never actually peeked into that directory nor read anything from them in the end.
It'll be easier to see once I made all the agent sessions public, and I might be wrong (I didn't observe the agent at all times), but seems the agent never used though.
That'll be dope. The tokens used (input,output,total) are actually saved within codex's jsonl files.
But I think this is one of those experiments that I need to put a halt to sooner rather than later, because the scope can always grow, my mind really likes those sorts of projects, and I don't have the time for that right now :)
You're not the only one to say this, maybe there is a value in a minimal HTML+CSS browser that still works with the modern (non-JS using) web, although I'm not sure how much.
Another idea I had, was to pile another experiment on top of this one, more about "N humans + N agents = one browser", in a collaborative fashion, lets see if that ends up happening :)
I'll keep them in mind for the future, who knows, maybe some interesting iteration could be done on what's been made so far.
I wonder if you've looked into what it would take to implement accessibility while maintaining your no-Rust-dependencies rule. On Windows and macOS, it's straightforward enough to implement UI Automation and the Cocoa NSAccessibility protocols respectively. On Unix/X11, as I see it, your options are:
1. Implement AT-SPI with a new from-scratch D-Bus implementation.
2. Implement AT-SPI with one of the D-Bus C libraries (GLib, libdbus, or sdbus).
3. Use GTK, or maybe Qt.
I know it's a little apples-and-oranges (you and the agent wouldn't produce the exact same thing), but I'm not asking because I'm interested in the man-hour savings. Rather, I want to get a perspective on what kind of expertise went into the guidance (without having to read all the guidance and be familiar with browser implementation myself). "How long this would have taken the author" seems like one possible proxy for "how much pre-existing experience went into this agent's guidance".
If you paste https://github.com/embedding-shapes/one-agent-one-browser into the "GitHub Repository" tab it estimates 4.58 person-years and $618,599 by year-2000 standards, or 5.61 years and $1,381,079 according to my very non-trustworthy 2025 estimate upgrade.
I don't think I'd be able to do this on my own. Not that I don't know Rust, but because I don't know X11 (nor macOS or Windows) well enough to even know where to begin.
I've been a Linux user for almost two decades, so I know my way around my system, but never developed X11 applications or anything, I'm mostly a web developer who jumped around various roles through the years. Spent a lot of time caring deeply about testing, infrastructure, architecture/design and communication between humans, might have given me a slight edge in programming together with agents.
That's why taking a step back and seeing what's actually hard in the process and bad with the output, felt like it made more sense to chase after, rather than anything else.
FWIW I ran your binary and was pleasantly surprised, but my low expectations probably helped ;)
The next challenge I think would be to prove that no reference implementation code leaked into the produced code. And finally, this being the work product of an AI process you can't claim copyright, but someone else could claim infringement so beware of that little loophole.
I think the focus with LLM-assisted coding for me has been just that, assisted coding, not trying to replace whole people. It's still me and my ideas driving (and my "Good Taste", explained here: https://emsh.cat/good-taste/), the LLM do all the things I find more boring.
> prove that no reference implementation code leaked into the produced code
Hmm, yeah, I'm not 100% sure how to approach this, open to ideas. Basic comparing text feels like it'd be too dumb, using an LLM for it might work, letting it reference other codebase perhaps. Honestly, don't know how I'd do that.
> And finally, this being the work product of an AI process you can't claim copyright, but someone else could claim infringement so beware of that little loophole.
Good point to be aware of, and I guess I by instinct didn't actually add any license to this project. I thought of adding MIT as I usually do, but I didn't actually make any of this so ended up not assigning any license. Worst case scenario, I guess most jurisdictions would deem either no copyright or that I (implicitly) hold copyright. Guess we'll take that if we get there :)