add explainer for the declarative api

[MiguelsPizza]

This draft proposal outlines a declarative WebMCP API that enables web pages to expose tools via HTML, using minimal attributes like tool-name and standard form semantics. Currently, it's a compilation of my notes and ideas from developing this approach, and I'm sharing it to gather feedback before the September 18th working group meeting. I'm particularly interested in your thoughts on the open questions (e.g., JSON vs. HTML responses, elicitation flows), tradeoffs, and overall API design.

The proposed API was shaped by building a real application and polyfill during the MCP enterprise hackathon, where our team successfully implemented it (and took home the win, which was exciting validation!). You can see a video of a Rails app using declarative WebMCP tools to enable complex browser automation without client-side JavaScript: link.

Based on your feedback, I'll refine this draft to align more closely with the structured format and narrative style of other explainers in the repo.

[bwalderman]

This is great work. I do have one general question. Was reusing ARIA attributes instead of introducing new tool-* attributes considered?

There are already attributes such as aria-label and aria-description and others for labelling and describing elements and so it might be helpful to define WebMCP mappings/behaviors for these instead of introducing entirely new HTML attributes.

One benefit of using these existing attributes is that they are also surfaced in native accessibility APIs, so assistive tools that already use these APIs to access the page's accessibility tree would be able to access WebMCP tools declarations as well.