Every SEO audit tool I've built takes a URL. Which means you have to deploy the HTML first, then audit it, then fix, then deploy again. Round-trip per change.
The HTML Sandbox Playground cuts the round-trip. You paste HTML, the audit updates live as you type. No deploy required. No URL needed.
What it does
- Paste a full HTML document (or a fragment with basic wrapping)
- The audit panel below updates within 350ms of you stopping typing
- Shows counts of pass / warn / fail + per-check findings
- Save named drafts to localStorage (up to 20)
- Load any saved draft with one click
What it audits
Per paste:
<title>present and populated<meta name="description">present- Exactly one
<h1> <link rel="canonical">present<html lang>attribute<meta name="viewport">for mobile- Open Graph tags (og:title minimum)
- Twitter Card tags
- JSON-LD blocks (count + valid JSON +
@typelist) - Images with missing
alt - Main-content word count (≥300 good, 100-299 warn, <100 fail)
<a href="javascript:">anchors (CSP blocker)- Inline event handlers like
onclick=(CSP blocker)
It's not a full Lighthouse replacement. It's the last-mile checklist before you hit deploy.
Why live-audit instead of button-triggered
Live feedback changes editing behavior. When you can see the "fail: Missing meta description" line disappear the moment you paste in a <meta> tag, the feedback loop becomes instant. Better than clicking Run each time.
The audit runs on a 350ms debounce after you stop typing — fast enough to feel responsive, slow enough to avoid running the parser on every keystroke.
Saved drafts — version history for your head tag
The localStorage-backed drafts are the underrated feature. Scenario:
- Paste your current landing page HTML. Save as "landing-v1".
- Make 10 edits — add schema, tighten meta description, fix H1 hierarchy.
- Save as "landing-v2".
- Want to compare? Click "Load" on v1, note the finding counts. Click "Load" on v2, note the new counts. See exactly what improved.
Crude but works. For serious version control, use git. For rapid iteration on a single HTML file, localStorage is faster.
The paired tool — HTML Sandbox + Action Kanban together
Run audit findings through the Kanban (bulk import). Fix findings in the Sandbox (live feedback). Move Kanban cards to Done. Deploy.
That's the full loop:
Audit (Mega SEO Analyzer) → Findings → Kanban (tracking) → Sandbox (fixing) → Deploy → Re-audit
All four tools browser-only, all data in localStorage, zero recurring cost.
Load sample button
The Load Sample button drops a minimally-correct HTML template into the textarea. Use it as a starting skeleton when you want to test additions against a known-good baseline.
Limits
- Static HTML only. Doesn't render JS or execute scripts. If your site depends on SPA hydration, use the Rendered DOM Paste Audit instead.
- No CSS evaluation. Can't detect visual issues like mobile viewport crowding or color contrast. For those, use the full Mega SEO Analyzer or a browser accessibility extension.
- No live fetch of external resources. If your schema references an
@idthat's an external URL, the sandbox doesn't follow it. - localStorage cap. 20 drafts max; each should be under ~200KB HTML. Larger pages: test in smaller chunks.
Why I built this now instead of first
Because I didn't realize I needed it until I'd built 14 other tools, each one requiring me to deploy HTML somewhere and test against it. Building the sandbox last is the obvious in-retrospect move.
Pair it with
- Action-Tracking Kanban — paired release, tracks which findings you've addressed
- Code-Diff Patch Generator — produces HTML patches you paste straight into the sandbox to verify before shipping
- Mega SEO Analyzer v2 — once the HTML is live, run the full audit against the URL
Related reading
- Action-Tracking Kanban walkthrough — the paired tool
- Code-Diff Patch Generator
- Mega SEO Analyzer v2 — paid-tool parity
Fact-check notes and sources
- DOMParser API: MDN DOMParser.
- localStorage quota: MDN Web Storage API.
- Debounced input handling pattern: standard JS
setTimeout+clearTimeout. - CodePen / JSFiddle reference: CodePen pricing, JSFiddle.
This post is informational. Mentions of CodePen, JSFiddle, StackBlitz, Linear, and similar products are nominative fair use. No affiliation is implied.
