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When this goes wrong, the browser usually keeps rendering. That is what makes the bug easy to miss: recovery can hide a broken source document. In this lesson, the focus is disclosure, menus, dialogs, and stateful markup. That matters because HTML is interpreted by a parser and then exposed to browser features such as navigation, accessibility tooling, form controls, media handling, and script APIs. A correct-looking screenshot can still hide a poor document model.
<button aria-expanded="false" aria-controls="menu">Menu</button><ul id="menu">...</ul>
Start with the smallest meaningful example. The working snippet demonstrates the core syntax, but the important part is what each piece contributes: the element supplies semantics or behavior, attributes refine that meaning, and surrounding structure gives the browser context. Don't add markup just because a framework or copied example happens to include it.
Implementation note: Start from semantic HTML, define the state explicitly, then wire behavior around it. Keep the browser behavior observable while you work.
Don't confuse browser-side convenience with a security boundary. The markup in building interactive components with html should improve behavior, but authoritative validation and policy belong at the trust boundary that processes the request.
<!-- Contrast the weak version --><div>Building Interactive Components with HTML used as a generic container</div>
Now put the same idea under pressure. A realistic failure is: Start from semantic HTML, define the state explicitly, then wire behavior around it. The browser may recover, ignore an invalid combination, or leave the user with a control that appears visually fine but behaves differently than expected. That is why source inspection and keyboard testing belong in the workflow.
<section aria-labelledby="topic-title"><h2 id="topic-title">Building Interactive Components with HTML</h2><p>Use the element that matches the content's meaning.</p></section>
For this topic, the edge case is worth remembering: HTML has legacy parsing and compatibility behavior, so invalid markup does not always fail loudly. If you are debugging a strange result, reduce the page to one element at a time, validate the source, and inspect the resulting DOM. Small reproductions beat guessing at the full application.
When you debug this in a real project, inspect both the source and the parsed DOM. The two are related, but they are not always identical.
Source-reading note: the exact semantics here come from the HTML element/attribute you chose, not its CSS appearance.
Field tip: for building interactive components with html, test both the ordinary path and the first case that makes your assumptions false.
Finish the lesson once you have worked through the material. This awards ★ 20 XP.