HTML Page Auditor (Lighthouse-style)

This tool runs the kind of checks Google’s Lighthouse runs, but on the parts of a page that live in the markup itself. You give it the HTML; it parses that HTML into an inert document with the browser’s built-in DOMParser — no scripts run, no images or stylesheets are fetched, nothing is rendered — and then it inspects the structure. From that it scores four categories the same way Lighthouse groups them: SEO (does the page have a title, a meta description, a viewport tag, a language, exactly one h1, a canonical link, and is it actually indexable rather than noindex?), Accessibility (do images have alt text, do form fields have labels, do buttons and links have an accessible name, are heading levels sane, is zoom left enabled?), Best Practices (is there a doctype and a charset, any deprecated tags, insecure http:// resources, or inline event handlers?), and a structural Performance pass.

It is honest about its one real limit: it cannot produce a timed performance score, and it cannot reach out and audit an arbitrary live URL. Measuring Largest Contentful Paint, Cumulative Layout Shift, and Total Blocking Time requires loading the page inside an instrumented headless browser, which only a tool like the real Lighthouse can do. And a page running in your browser is forbidden by the same-origin policy from reading another website’s HTML, so “type any URL” isn’t possible without a server proxy — which would break the promise that nothing you paste ever leaves your device. So you paste the source (or fetch a page on this same domain), and the Performance ring scores the structural signals that genuinely live in the markup: render-blocking scripts in the head, images missing width and height (a classic layout-shift cause), the number of render-blocking stylesheets, and overall document weight.

Every category is a weighted bundle of individual audits. A pass earns the audit’s full weight, a warning earns half, a failure earns nothing, and an audit that doesn’t apply — the image-alt check on a page with no images, say — is dropped from the maths entirely so it can’t unfairly drag a score down. The weighted total is scaled to 0–100 and coloured with the familiar Lighthouse bands: green at 90 and above, orange from 50 to 89, and red below 50. The audit list under the rings sorts failures and warnings to the top, each with a short explanation of what’s wrong and the fix, so the page becomes a punch-list you can work straight down. Fix an item, re-paste the updated HTML, and watch the ring climb.

There are six categories in all: the four from Lighthouse — SEO, Accessibility, Best Practices and Performance — plus a Security bundle (Subresource Integrity, iframe sandboxing, insecure forms, CSP-friendliness, risky JS sinks) and a Privacy bundle (third-party origins, known trackers, third-party fonts and embeds) that Lighthouse largely skips. The overall number is the average of the six rings, so a fast page that ships trackers gets an honest score rather than a perfect one.

A practical workflow: build or generate your page, paste it here (or fetch a same-origin URL) to clear the structural SEO, accessibility, security and privacy issues quickly and privately. Then hit Load & measure shift to load the page in a sandboxed frame and capture the real Core Web Vitals — CLS, LCP, page weight, request count, blocking time, and the exact elements that shifted — all without leaving your browser. For the full background on what each check means and why it matters, read How Web Page Auditing Works. Pair it with the Meta Tag Generator and Schema / JSON-LD Generator to fix the SEO findings, and the robots.txt Generator for crawl control.