It's at the top of the article, and also the correct explanation of such: In an HTML API, the definitions and protocols are in the HTML itself, and the tools

It's at the top of the article, and also the correct explanation of such: In an HTML API, the definitions and protocols are in the HTML itself, and the tools look in HTML for the configuration. HTML APIs usually consist of a certain class and attribute patterns that can be used on existing HTML. HTML is just markup. You're familiar with markup if you've ever used a hilighter for instance. To make it an API just have something parse that markup according to some well-defined set of rules and to do _stuff_. I can write a parser that accepts a single API call as it were: <p data-do-a-thing>Smith</p> Now I can feed that into the parser and in this API of mine a `p` tag with attribute `data-do-a-thing` renders the contents of the tag prefixed with `Hello, ` giving `Hello, Smith`. Viola an API via HTML.