Validation — Complete Guide
Validation — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of HTML Tutorial on Toolliyo Academy.
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HTML Tutorial · Lesson 27 of 100
Validation
Basics ✓ → Forms & semantics → APIs & performance → Projects
Forms & semantics · 2 — Forms, SEO, a11y · ~6 min · HTML — Forms & Validation
What is this?
Built-in HTML validation uses required, type, min/max, pattern, and minlength/maxlength before submit.
Why should you care?
MarkupVerse catches empty emails client-side — and still validates on the server.
See it live — copy this example
Save as demo.html and open in your browser, or use Run Example below.
<form>
<label>Email <input type="email" name="email" required></label>
<label>Username
<input name="user" required minlength="3" pattern="[a-z0-9_]+" title="lowercase letters, numbers, underscore">
</label>
<button>Create account</button>
</form>Run Example »
Edit the code below and click Run to see the result in Toolliyo’s live editor.
What happened?
- title helps explain pattern failures.
- novalidate disables native UI for custom flows.
- Never trust the client alone.
Practice next
- Add required email.
- pattern on username.
- Trigger invalid state.
- Add maxlength="32".
- Try novalidate + custom message.
Remember
required/type/pattern. Helpful title. Server still rules.
MarkupVerse signup checks
Empty email blocked by browser.
Outcome: Fewer bad requests hit the API.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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