Canonical URL
Why it matters
The canonical URL tells Google which version of a page you consider the original. Without it, Google makes its own determination. On a large site with multiple URL variations — www vs non-www, trailing slashes, URL parameters from tracking or filtering — that determination is often wrong.
Duplicate content is common and not inherently penalised. What matters is which version accumulates ranking signals. If you have three variations of the same page and each receives backlinks, Google may distribute signals across all three. The canonical tag consolidates them to the one version you specify.
For e-commerce sites, filtered category pages often produce hundreds of near-identical URLs. Without canonical tags pointing back to the base category page, each filter combination competes independently. Rankings stay weak even with a strong backlink profile.
Self-referential canonicals — where the canonical URL matches the page URL — are correct and intentional. They prevent external tools, CDNs, or proxies from accidentally suggesting to Google that some other URL is preferred.
How we check it
We read the href attribute of <link rel="canonical"> from the rendered DOM. We check for both <head> placement and any changes introduced by JavaScript.
If the canonical changes — pointing to a new URL, going blank, or appearing where none existed before — we flag it with before/after values.
What to watch for
A canonical that changes to point to a different page is one of the more damaging changes to miss. It silently transfers ranking signals to another URL. A canonical that disappears leaves you exposed to duplicate content problems.
Check the HTTP status code of the canonical target URL. If the canonical points to a redirect or a 404, it defeats the purpose.