Useful patterns for building HTML tools
simonwillison.net
simonwillison.net
My tool collection [0] is inspired by yours, with a handful of differences. I'm only at 53 tools at the moment.
What I did differently:
Hosted on Cloudflare Pages. This gives you preview URLs for pull requests out the box. This might be possible with Github Pages but I haven't checked. I've used Vercel for similar projects in the past. Cloudflare seems to have the odd failed build that needs a kick from their dashboard.
Some tools can make use of Workers/Functions for backend processing and secrets. I try to keep these to a minimum but they're occasionally useful.
I have an AGENTS.md that's updated with a Github action to automatically pull in Claude-style Skills from the .skills directory. I blogged about this pattern and am still waiting for a standard to evolve [2].
I have a base stylesheet that I instruct agents to pull in. This gives a bit of consistency and also let's them use Tailwind, which they'd seem to love.
[0] https://tools.dave.engineer/
Edit: come to think of it, I should revisit it now that everyone can vibe code. The sheet was to allow people to add to it, now maybe easier for me to take a message and ask an agent to update the html directly
Couple of unsolicited comments: first is that on mobile, the featured badge sits on top of the right facing arrow. Second is that the bubble level seems to be upside down? The bubble sinks rather than floats at least on my pixel
Sorry if this sounds overly critical, but what do you mean "only at 53 tools?" Was there a memo I missed about a competition to host LLM-built tools?
For anyone interested, to achieve synchronization I basically just use the https://github.com/google/diff-match-patch lib and save the patches in a db for each version with a version id. Then there's a generic JS file that I inject to uploaded HTML files that monkey patches the localstorage methods and optimistically updates the localstorage at the same time sending the diff to the server to save to the db.
The only drawback I can think of is that all of your commits are broadcast on a megaphone to the network firehose, but encryption can alleviate that somewhat.
In this type of scenario there are a lot considerations to be made though, specifically since you can't use CRDT's to handle concurrent updates on the data you have to either 1) not allow offline use of the apps, 2) create a merge conflict resolving interface or 3) just overwrite all changes with the latest one.
Idk if people would be interested in this and I haven't been using my HTML tools for a while now, so it's just an idea, maybe someone else wants to work on.
That's what remoteStorage is for.
One problem I solved with this was a packer needed to scan a few (10-40) ids into his barcode scanner. It was not enough where pulling up their bulk-id-uploader program but also too tedious to go to some "number to barcode" website.
Turns out, barcodes can be made from a google font!
https://fonts.google.com/specimen/Libre+Barcode+39
You can just display a number using that font. Then hooked up a for-loop that's progressed by pressing the space bar: paste in IDs, scan first, space, scan next, repeat.
Personal tools seem like a reasonable place for happy path vibecoding given small blast radius and LLMs can do that sort of static page in front of python backend really well.
I've also been surprised how much active learning I'm doing despite specifically not look at code. Between the need to spec things out carefully (plan.md) and fast iteration loop it's been a huge boost. Having the LLM look at a plan.md and suggest improvements has lead to a lot of "oh I didn't think about that" learning on architecture and user requirements link.
Presumably much of that learning boost is because I'm a hobbyist tier programmer, guessing professionals wouldn't experience the same since they learned this via manual coding trial & error over years.
I can only speak for myself and my not-quite two decades of professional experience, but yes pretty much!
It’s neat to see that sped up for others though with lower stakes, though it’s not quite the same unless you prompt your agent to question you back a lot (Claude is much better at this in my experience)
The CDN approach works, but I don't love depending on some third-party service just so your app continues working. Instead, I like using vite with vite-plugin-singlefile. This lets you package your JS and CSS into a single HTML: https://www.npmjs.com/package/vite-plugin-singlefile
I found out about a new Python HTML parsing library - https://github.com/EmilStenstrom/justhtml - and wanted to try it out but I'm out without my laptop. So I had Claude Code for web build me a playground interface for trying it out: https://tools.simonwillison.net/justhtml
It loads the Python library using Pyodide and lets you try it out with a simple HTML UI.
The prompts I used are in this PR: https://github.com/simonw/tools/pull/156
Thank you.
In the case of JustHTML I've now been able to try it against a few different HTML documents, seen it do good pretty-printing, played with its CSS selector implementation and got a feel for its event-based streaming parser. I'm very impressed! I think I'll be using it in the future next time I need an HTML parser.
Until vibe coding came along, the ergonomics of a library were no less important than its functionality. But I understand how LLM assisted coding changes that perspective.
I'll go tend to my empty lawn now.
The idea is interesting, shame there is nothing for full stack like this, something like opinionated fossil-scm setup - which already has project management built in (for llm to use for its dev progress); together with backend and runtime state squashed inside single sqlite so you can create/delete them independently without a fuss.
More recently, I've found a lot of benefit from using the extended thinking mode in GPT-5 and -5.1. It tends to provide a fully functional and complete result from a zero-shot prompt. It's as close as I've gotten to pair programming with a (significantly) more experienced coder.
One functional example of that (with 30-50% of my own coding, reprompting and reviews) is my OntoGSN [1] research prototype. After a couple of weeks of work, it can handle different integration, reasoning and extension needs of people working in assurance, at least based on how I understood them. It's an example of a human-AI collab that I'm particularly proud of.
[1] Playground at w3id.org/OntoGSN/
I don't have a lot of public examples of this, but here's a larger project where I used this strategy for a relatively large app that has TypeScript annotations for easy VSCode use, Tailwind for design, and it even loads in huge libraries like the Monaco code editor etc, and it all just works quite well 100% statically:
HTML file: https://github.com/blixt/go-gittyup/blob/main/static/index.h...
Main entrypoint file: https://github.com/blixt/go-gittyup/blob/main/static/main.js
You could definitely build such a shell with Electron or Tauri, it punches a big hole in their security model, but you could do it.
I've also been using LLMs to create and maintain a "work assist" Chrome extension that I load unpacked from a local directory. Whenever I notice a minor pain point, I get the LLM to quickly implement a remedy. For example, I usually have several browser tabs open for Jira, and they all have the same company logo as the favicon, so my Chrome extension changes the favicon to be the issue type icon (e.g. Bug, Story, etc) when the page loads. It saves a little time when I'm looking for a specific ticket I've already opened.
I think especially in context of software that is complex and takes a long time to master, this could be the next breakthrough. Instead of paths-to-goal being buried in sequences of menus and config panels, workflow pathways would be invocable with plain language.
This really showcases the power of the single page apps and why web will be always ahead of native for this kind of Swiss Army Knife tools.
With LLMs, it gets ridiculously easy to “develop” (generate) those too.
One tool I'd really like to see in this format is a simple "turn the background of this PNG to transparent". Models still refuse to follow the instruction to create transparent backgrounds for logos they create, and I often have to look for other tools doing this as post-processing.
It's possible that this is too complicated for the "few hundred lines of js" code envelope, though.
Build transparent-png.html - a tool that lets you open any image and then click on colors within that image to make them transparent - showing a preview of the resulting PNG against a checkerboard pattern and optional against other selected background colors below, plus a download PNG option
It should also accept pasted images
(I’m not actually kidding)
Seriously, though, I think this solves a nicely framed simpler problem. I was thinking about a more general tool, but that's genuinely hard (you'll need heavy CV algorithms or a special ML model to detect what is background what what isn't).
To be honest, what you built here is probably sufficient anyway, because the models are better at obeying "create a white background" or "create a 0xffffff background" than "transparent", so this tool can post-process to what's needed.
When asked for "transparent", I've had a model generate a fake checkerboard pattern of gray colors to imitate how viewers render transparent areas :-) For this kind of nonsense, the transparent-png tool wouldn't do!
Things like styling buttons, responsiveness, and so on are better solved once.
A good rule of thumb is: if the shared CSS fails to load, page still fully works but it might be uglier (weird fonts, etc). That's a reasonable rule for proper isolation (tools remain simple to understand, code remains reusable, etc).
I love the idea of self-contained tools, but you're already using CDNs. Having a shared CSS wouldn't hurt and actually make the tools better.
I would go as far as having a shared JS too (same idea, works if it doesn't load).
That's essentially what I did in https://alganet.github.io/spiral/ (also vibe coded).
Each spiral is mostly independent. You can go ahead and delete the shared CSS from the <head>, they still work and don't break funcionality. However, by having the shared CSS I made them consistent, made them friendly to phone users and so on.
It's been fun collecting a bunch of inconsistent tool designs just to see how the different models behave, plus occasionally I go for something with a topical theme like https://tools.simonwillison.net/terminal-to-html or https://tools.simonwillison.net/new-yorker-style - but a little more consistency could be nice.
Not only for the user, but it makes sense for the process of making the tools as well.
If I left the agent for itself, it often come up with outrageous styles and I need to prompt it for something more sober.
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You can do a lot with just CSS. I restored this 2009 project of mine just now:
https://alganet.github.io/ghiaweb/
It still works (minor misalignments though), all HTML is pure (no class=, no css=, no <div>). The global CSS does everything: the forms, the drop-down menus, etc.
Nowadays, we can do even better, no build step or anything like that.
- Shell scripts, AppleScripts, etc. that I trigger from Alfred
- Obsidian plugins
- The occasional Emacs Lisp function
They serve a similar purpose for me as OP's HTML Tools, in the sense that they let me automate a small part of my workflow that I wouldn't otherwise have automated. If I have to choose between writing AppleScript and just doing something manually, I'll pick doing something manually 100% of the time. But if I can just ask an LLM to write the automation for me and then test it in a bunch of different scenarios, the choice becomes much easier.
After reading this post, I really want to try moving some of my automations to the web. Using HTML/JS/CSS for some of these tools will let me solve a whole different set of problems. E.g. I could more easily build automations for the non-techy folks in my family instead of just keeping them to myself.
AppleScript’s human readable language lulls you in this false sense of security that you can wing it and everything will just work out. This is simply not the case, it is a very quirky language and it helps to read a book to get the right mental model.
The second thing that helped was getting AppleScript debugger from Late Night Software. They recently decided to no longer develop it and release it for free on their site. It’s worth getting if you haven’t done so already.
Not sure why, but the moment the file is split into files and subfolders, coding agents tend to do a lot more changes that what is absolutely necessary. That way a single html file wins!
LLMs are generating app for an idea that can fit in few hundred lines of html/js. Had an idea that what if brushes were dithered in a painting tool and made a dithered painting tool. https://github.com/SMUsamaShah/dither-painter
These tools and code are ephemeral though. You don't need to use mine for example. Just ask the LLM of the time to implement the idea and in most cases it will work fine.
Reviewing data in Excel is painful, especially when answers are in HTML or Markdown, because you don’t get proper rendering. Building small, custom tools that reduce the friction of reviewing data makes life much easier and more pleasant. These days, I use Claude Code for Web to build most of these apps, and they are deployed on Vercel.
I wonder if packaging the results as web components would be the next logical step.
No. You can vendor these scripts & host them 1st party so you aren’t leaking data to these CDNs or risk users not actually getting the scripts. It isn’t like CDNs give you a performance boost anymore.
I'll vendor and self-host for my professional projects, but for these small experimental utilities I've stopped caring.
This is what CDNs should be used for at this time—or for fetching the scripts to vendor. That’s fine, but recommending I don’t think is the best call since one folk’s experimental utility will inevitably get released into production—often not even at fault of the utility’s maker. When I use CDNs like this, there are <!-- WARNING … --> around the code just in case someone were to run with it, along with adding the integrity attribute.
it does something like this
and connects through BLE
As if your steady stream of learning-in-public experiments and insights weren't generous enough. Seriously, massive kudos for sharing all the details.
They have a library of sample apps you can edit but I wish they included the prompts and history to build each since I generally can’t get large apps to work - after a while the I’ll just produces more bugs as complexity grows. But I’m also a bad vibe coder and never read the code so entirely my fault :)
It may well do that, but it's not earned my trust yet!
I have a Vue3 started template I host at https://http://vue-template.spaghet.me/ and all I have to do is curl and I'm ready to go.
Showcase:
https://timer.spaghet.me/ https://colors.spaghet.me/ https://box.spaghet.me/ https://talk.spaghet.me/ https://farming.ope.cool https://stitch.ope.cool https://draw.ope.cool https://walz.ope.cool
Create PDFs from images, a Wordle hint/solver, or a classic DVD screensaver. Lots of stuff.
I list them at https://client-side.app/
I use indexedDB for it and will use sqlite if I start to get more serious data needs.
I haven’t found too many issues with loading React and Babel from a CDN. I find React easier to read than straight HTML/JS. I find it more annoying to code in but seeing what state is needed in what components is a pleasant reading experience for me with single file tools.
I'm with you though, personally react is a acceleration mechanism for me because I often find existing well built components already. I don't built the same thing as the author though.
I could do an authentication protected one that only I could access though...
I tend to make them as Python servers which serve plain html/js/css with web components. I know this is a bit more complicated than just having a single html file with inline js and css, but the tools I made were a bit too complicated for the LLMs to get just right, and separating out the logic into separate js files as web components made it easy for me to fix the logic myself. I also deliberately prompted the LLMs to avoid React because adding I didn't want to need a build step.
The only one I actually still use is the TODO app I made: https://github.com/cooljoseph1/todo-app It stores everything in a JSON file, and you can have multiple TODO lists at once by specifying that JSON file when you launch it.
For better/worse, and whether completely so or not, the time of the professional keyboard-driven mechanical logic problem solver may simply have just come and gone in ~4 generations (70 years?).
By 2050 it may be more or less as niche as it was in 1950??
Personally, I find the relative lack of awareness and attention on the human aspect of it all a bit disappointing. Being caught in the tides of history is a thing, and can be a tough experience, worthy of discourse. And causing and even forcing these tides isn’t necessarily a desirable thing, maybe?
Beyond that, mapping out the different spaces that are brought to light with such movements (eg, the various sets of values that may drive one and the various ways that may be applied to different realities) would also certainly be valuable.
But alas, “productivity” rules I guess.
I guess if what you really want is only the finished product and nothing else, churning it out as quickly as possible with AI and not caring about the implementation could work for you. But it would take the fun out of it for me.
Sadly my career may eventually head in that direction. At least I'll always have a hobby to enjoy.
Same here! That's why I'm having so much fun building nearly 100 of them in a year.
The difference here is that I didn't have to type out all of the code by hand.
This issue is relevant if your app's functionality includes the user changing the contents of the file and re-saving as a new file.
also sad, that XHTML was abandoned.