Markdown has become the dominant file format used by agents to communicate with us. It’s simple, portable, has some rich text capability and is easy for you to edit. Claude has even gotten surprisingly good at using ASCII to make diagrams inside of markdown files. But as agents have become more and more powerful, I have felt that markdown has become a restricting format. I find it difficult to read a markdown file of more than a hundred lines. I want richer visualizations, color and diagrams and I want to be able to share them easily. I'm also increasingly not editing these files myself, but using them as specs, reference files, brainstorming outputs, etc. When I do make edits, I’m usually prompting Claude to edit them, which removes one of markdown’s largest benefits. I’ve started preferring HTML as an output format instead of Markdown and increasingly see this being used by others on the Claude Code team, this is why. (if you want to start with some examples, you can see a bunch here: https://thariqs.github.io/html-effectiveness, just be sure to come back and read more about why) Why HTML? Information Density HTML can convey much richer information compared to markdown. It can of course do simple document structure like headers and formatting, but it can also represent all sorts of other information such as: Tabular data using tables Design data with CSS Illustrations with SVG Code snippets with script tags Interactions using HTML elements with javascript + CSS Workflows using SVG and HTML Spatial data using absolute positions and canvases Images using image tags I would go so far as to say that there is almost no set of information that Claude can read that you cannot fairly efficiently represent with HTML. This makes it a highly efficient way for the model to communicate in-depth information to you and for you to revie wit. I’ve found that in the absence of being able to do this, the model may do more inefficient things in markdown like ASCII diagrams or, my favorite, estimating colors with unicode characters like in this screenshot from Claude Code. Visual Clarity & Ease of Reading As Claude is able to do more complex work, it is also writing larger and larger specs and plans. In practice, I've found I tend to not actually read more than a 100-line markdown file, and I certainly am not able to get anyone else in my organization to read it. But HTML documents are much easier to read, Claude can organize the structure visually to be ideal to navigate with tabs, illustrations, links, etc. It can even be mobile responsive so you can read it differently based on your form factor. Ease of Sharing Markdown files are fairly hard to share since most browsers do not render them natively well. You often have to add them as attachments to emails or messages. With HTML, as long as you upload the file (for example to S3), you can share the link easily. Your colleagues can open it wherever they wish and easily reference it. The chance of someone actually reading your spec, report or PR writeup is much much higher if it’s in HTML. Two-way Interaction HTML can allow you to interact with the document, for example you might want to ask it to add sliders or knobs to adjust a design or allow you to tweak different options in the algorithm to see what happens. You can also ask it to let you copy these changes into a prompt to paste back into Claude Code. Read more about my playgrounds post to see examples of this two way interaction: https://x.com/trq212/status/2017024445244924382 Data Ingestion Why use Claude Code to make HTML files instead of ClaudeAI or Claude Design for example? One of the biggest reasons is all the context Claude Code can ingest. For example, when writing this article, I asked Claude Code to read through my code folder and find all the HTML files I’ve generated, group and categorize them and then make an HTML file with all diagrams representing each type. The diagrams you see in this article are a direct result of that. Besides the file system, Claude Code can find additional context using your MCPs (like Slack, Linear, etc.), your web browser (with Claude in Chrome), your git history, etc. It’s Joyful Making HTML documents with Claude is just more fun and makes me feel more involved and invested in the creation, and that by itself is enough. How to Get Started I’m a little bit afraid that people will read this article and turn it into a /html skill or something. While there might be some value in that, I want to emphasize that you don’t need to do much to get Claude to do this. You can just ask it to “make a HTML file” or “make a HTML artifact”. The trick is knowing what you want the artifact to do and how you might use it. You may over time make a skill, but for now I’d suggest just prompting from scratch to get a hang of how to use it in different cases. Use Cases To make this more concrete, I’ve made many different HTML files for different use cases. You can view all of them here: https