Posted by paulirish 3 hours ago
Models know about some of these by name, but consistently won't default to using the platform-native solution and instead prefer the incumbent <div>+JS based pattern. We started an effort in Chrome to supplement coding agents with this key domain knowledge and we're shipping our 1.0 today, in collaboration with Firefox, MDN, and Edge.
200+ guides (~1.7MB) of curated, hand-crafted implementation best-practices from web experts, updated as the web evolves. (Open source, obv, so.. we'll welcome your bikeshedding ;) Readable versions here: [1]
I've spent the last 9 months obsessing over how to ship this as an agent skill + (Agentic RAG) CLI without blowing up your context window. (Or cedeing activation mechanics to MCP dynamic tool search). The skill description is minmaxed for activating on relevant frontend tasks. The RAG work I'm pretty proud of… originally a 500mb native stack down to a ~7MB pure-JS package, ~10ms search on your CPU.
We just introduced a deterministic 'legacy-scan' (trained on 30k github repos) to identify old 'n dusted patterns that your agent can modernize. (92% avg precision on held-out repos.)
People make a lot of claims about what coding agents are good/bad at. We wanted receipts, so we invested heavily in eval infra [2] to help us hill-climb, but also to identify when knowledge is common enough we can prune our guides. (Ofc, some of the things we thought AI didn't know turned out to be wrong :) We run codex/claude/agy against 200 tasks without the skill and then with.. In total ~1,700 playwright/AST-based assertions. Agents go from 55% pass unguided to 92% guided.
Try it out:
DISABLE_TELEMETRY=1 npx modern-web-guidance@latest install
DISABLE_TELEMETRY=1 npx modern-web-guidance@latest search "animate dialog modal"
DISABLE_TELEMETRY=1 npx modern-web-guidance@latest legacy-scan ./src
Longer write-up on the 1.0: https://developer.chrome.com/blog/modern-web-guidance-v1
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0: https://news.ycombinator.com/item?id=50416456
1: https://developer.chrome.com/docs/modern-web-guidance/guides
CSS @function
CSS calc-size()
CSS interpolate-size
CSS corner-shape
CSS border-shape
CSS flex-wrap: balance
CSS text-fit
CSS text-decoration-skip-spaces
CSS scroll-axis-lock
CSS scroll-target-group
CSS :target-current
CSS scroll-initial-target
CSS container scroll-state queries
scrollsnapchange event
scrollsnapchanging event
HTML focusgroup
Element.startViewTransition()
CSS view-transition-scope
Interest Invokers (interestfor)
LanguageModel (Prompt API)
LanguageDetector
Translator
Summarizer API
Window.fetchLater()
Speculation Rules API
Long Animation Frames API
VisibilityStateEntry
PerformanceSoftNavigation
InteractionContentfulPaint
Connection-Allowlist
CSS url() integrity() modifier
HTML meta name="text-scale"
HTML template for
HTML processing instructions
PasswordCredential
Promise-returning scroll methods
Web App Origin Migration
Local Network Access permissions
Request.targetAddressSpace
beforeinstallprompt event
To be clear, the skills document how to use these features to solve common use cases, and then they give your agent a full browser support table for any APIs mentioned that are not Baseline Widely available -- and then your agent can decide whether to use them or not, based on your site's browser support requirements.
In most of these cases a cross-browser fallback is also documented.
"DO use fetchLater()..."
"Your own code MUST call fetchLater() directly..."
https://github.com/GoogleChrome/modern-web-guidance/blob/mai...
But yes, the skills do document compatibility and fallbacks well.
The mental model we've been using on the team is:
If you hired an expert web development consultant to build X, what would they recommend as a best practice? When does it make sense to use the new API (because it provides real benefits) vs. when does it make sense to wait until it's available everywhere?
To use your `fetchLater()` example, this is a case where the `fetchLater()` API legitimately solves a very real problem for people trying to build accurate analytics or telemetry features, see: https://nicj.net/beaconing-in-practice/
And lots of companies actually are already using `fetchLater()` in production now because of its increased reliability.
That index page should link to plain markdown versions of all of the documents, and should be designed for search with grep to find the right answers.
That way I don't have to install anything at all - provided my agent has the ability to make HTTP requests I can tell it "Look for tips on https://googlechrome.github.io/modern-web-guidance-src/skill..." and it will fetch the index, then navigate to the most relevant documents - no MCP or npm or installation required at all.
(we could make this clearer though)
We'll make this happen. :)
It feels that something like this would be the holy grail of agents. A small agent that has only the minimum necessary built in knowledge. The rest, it just retrieves on the fly when it needs (+ caching)
It’s like storing pointers/links instead of the contents
But yeah.. big corpus of valuable context × minimal agent footprint = happy place.
Came up with a good list for my project
https://github.com/GoogleChrome/modern-web-guidance-src/blob...
And now this just fell in my lap! Some of the arguments I wanted to make seem to be provide here. Didn't have a chance to take a proper look at it, but it looks very promising and nice!
Thank you!
So I'll take a 1-line CSS solution over 60 lines of ResizeObserver/setTimeout business. But.. best practices for frontend isn't just the code.. it's ensuring quality UX, that keyboard navigability works, that layout doesn't shift on users.
Browser-specific implementation gotchas tend to take about a decade to iron out for reasons that never have anything to do with code quality. Users are fickle and so are browser vendors, but that's a different topic.
At the end of the day, there's just no escaping lots of manual testing across device and browser combinations if your goal is a polished result and not just something that "works".
AI has done nothing to make this problem go away. If anything, it's made it worse because people don't even want to look at the code now!
I've recently finished a browser game specifically targeting an old iPad Air 1 we had lying around the house.
I had a working version after a weekend, but it took me a month to make it run smoothly on an over 10 years old mobile device. And the most recent browser available is almost 10 years old.
Surprisingly I didn't missed too many "modern" JS features, as these were mostly functional programming oriented which doesn't play well with optimization anyways, and the ES6 was more than enough as a programming platform.
Anyway, what I'm trying to say is that "the best practice" should be making your apps fast and accessible - as much as you can without having to sacrifice something you value - instead of chasing the latest and shiniest.
Don't be a fashion driven developer. :)
This is a misconception. Have your favorite agent summarize this video for you: https://www.youtube.com/live/zcLPGC-tvgk?si=YR68aHlFESD053bh
But the fun part is that all of the proper software engineering practices are aimed at lowering amount of context a developer has to keep in his mind. So it directly applies to agent context. Less verbose code, less tokens to burn, separation of concerns, lower chance agent breaks something unrelated etc.
>This is a misconception. Have your favorite agent *summarize this video for you*: https://www.youtube.com/live/zcLPGC-tvgk?si=YR68aHlFESD053bh
I can't tell if you're joking here
The more "legacy code" is getting written everyday the more the web will be forced to remain backwards compatible...
This is exactly what I need when I need it. The amount of pull requests I reviewed with a mess of codes that can be switched to 2 lines of modern features made me write such tools myself, only half assed since I have other things to do.