No canonical link

Why it matters

The same page is usually reachable at several addresses: with and without www, with a trailing slash, with ?utm_source=… from a newsletter. Without a canonical link, search engines choose which address to show and may split the page's standing between the copies. One line tells them which address is the real one.

Source: How to specify a canonical URL. Google treats it as a strong hint, not a requirement, which is why this is a warning.

How to fix it

Add this inside <head>, with the page's own full address (the one you want in search results):

<link rel="canonical" href="https://www.example.com/this-page" />

Most CMSs and SEO plugins add it for you once switched on.

Examples

These are the rule's own test pages, and what it says about each. They are run on every change to the rule book.

<html lang="en">
<title>A</title>
<link rel="alternate" href="/feed">

WarningThe page doesn't name its canonical address, so search engines pick one for you.

fail-none

<html lang="en">
<title>A</title>

link: <https://www.example.com/>; rel="canonical", </style.css>; rel=preload

PassedThe page names its canonical address.

pass-header

<html lang="en">
<title>A</title>
<link rel="canonical" href="https://www.example.com/">

PassedThe page names its canonical address.

pass-link