Beginner to Principal
HTML Mastery — From First Page to Web Standards Engineer
From your first element to parser internals and principal-level review. Every example renders in a sandboxed preview, shows the real DOM tree the browser builds, and was verified by the conformance checker before it shipped.
The course
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Common questions
Do I need to install anything to take this course?
No. Markup renders in a fully sandboxed frame inside the page, and the DOM tree view uses your browser's own parser. Nothing is installed or sent anywhere.
Are the examples actually valid HTML?
Yes. Every example, worked solution and common-mistake pane was checked with the Nu Html Checker — the validator behind validator.w3.org — and where a lesson shows a validator verdict, that is the checker's genuine output.
Is the HTML course free?
Yes. All 70 modules, the graded exercises and the timed drill are free and need no account.
Are the exercises actually graded?
Yes. Your markup is parsed and checked against 279 assertions — some shown, some hidden. The checks inspect the DOM you produce, never the text of your code, so any correct solution passes.
Does the in-page lab run the full validator?
No, and it says so. Its Checks tab runs structural DOM checks (alt text, heading order, labels, duplicate ids, link safety). The full conformance verdicts in the lessons come from the build-time checker.