JSFiddle and CodePen are great for JS but dont check SEO fundamentals. This sandbox audits your HTML live as you type - title, meta, canonical, OG, Twitter, JSON-LD, alt, word count, inline handlers. Save drafts locally, compare revisions, ship confident HTML.
Read the story behind this tool: Why I built this - pre-deploy SEO feedback in the browser.
Related: SEO Action Kanban (paired) / Rendered DOM Paste Audit