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HTML Document Structure & DOCTYPE

HTML Document Structure & DOCTYPE

WebContainer
0 XP
0d streak
Concept
Core Principles
Code Example
4Knowledge Check
5Hands-On Drill
60%
Lesson 1 · 7 min

HTML Document Structure & DOCTYPE

HTML is the foundation of every web application. Correct document structure ensures cross-browser compatibility, proper SEO indexing, and screen reader accessibility from the very first line of markup.

In this lesson

STEP 1

Every HTML document starts with a DOCTYPE declaration that triggers standards mode in the browser.

STEP 2

The lang attribute on <html> tells screen readers which language to use for pronunciation.

STEP 3

Charset must be the first tag in <head> to prevent encoding-based security issues.

Interactive Example

A minimal valid HTML document

Interactive Editor
typescript
Loading...
Output Console
Modify code and click "Run Code" (or ⌘+Enter) to evaluate output in real time.
What happens here?
  1. Start with <!DOCTYPE html> — no leading spaces, no quotes around html.
  2. Set lang='en' (or appropriate BCP 47 language tag).
  3. Place charset before title to prevent charset-switching attacks.

Quick check

What happens if you omit the DOCTYPE declaration?

Next: Phase 2 Hands-On Drill

Ready to write code?

Solve the coding challenge in the in-browser IDE with live visible and hidden specification tests.