Plumbtern: On-Page SEO Checker by Zydeno

On-page SEO checker extension for the page you have open. Click the toolbar button and Plumbtern lists what needs attention, most serious first, each remark with one sentence on why it matters. There is no score out of 100: a page either has the problem or it doesn't.

What it reads:
• Title and meta description, with characters, approximate pixel width and a guide range
• Headings H1 to H6 as an outline, with extra H1s, skipped levels and empty headings marked
• Canonical: absolute address, repeated tags, relative addresses, mismatch with this page
• Robots tags, with a clear warning at the top when a page is set to noindex or nofollow
• Hreflang: invalid and repeated codes, missing x-default, and whether the page lists itself
• Open Graph and social card tags: what's there, what's absent, what's relative
• Images: with alt, no alt, empty alt and decorative, each listed with its address
• Links: internal, external, nofollow, sponsored, ugc and links without text
• Word count of visible text, including languages written without spaces, like Japanese and Chinese
• Language, charset, viewport and JSON-LD types

Useful around the work:
• Copy the report as plain text or Markdown and paste it into a ticket or an email to a client
• Save a page as a baseline, then see what changed after your edits
• Select a heading, image or link to scroll to it on the page

It stays out of your pages. Plumbtern has no access to a site until you click, and asks for no access to all sites. It adds no toolbar, styles or scripts to the page, so dashboards and editors keep working as usual. The only exception is the two-second outline when you ask to see an element. Works offline in Firefox and Opera. Your data stays in your browser. No account, no network requests, no analytics.

What it doesn't do: it reads the page as it is now, so HTTP headers, robots.txt, broken links and other pages aren't checked. Length guides are common practice, not search engine rules, and passing them doesn't promise any ranking.

The interface and report are available in 16 languages, with light and dark themes and full keyboard control.

Permissions: activeTab gives access only to the tab you clicked on, only after the click. scripting runs the reading function there, and the outline when you ask for it. storage keeps your language, theme and saved baselines.