Fill in the blanks. When you want to use _____, make sure they are placed in plain HTML/X-robots tags. _____ injected by JavaScript are considered less reliable, and the chances are that Google will ignore them.

The correct answer is "Canonical tags." Canonical tags are crucial for SEO as they tell search engines which version of a page is the preferred one, preventing duplicate content issues. For Google to reliably process canonical tags, they should be placed directly in the HTML of the page or within the HTTP header (X-Robots-Tag). If canonical tags are injected via JavaScript, Google's ability to discover and process them becomes less reliable. Google's crawlers may not always execute JavaScript, or they might do so with a delay, leading to the canonicalization signals being missed or misinterpreted. Hreflang tags are used for international SEO to indicate the language and geographical targeting of a page, not for canonicalization. Rel=amp HTML tags are used to link to Accelerated Mobile Pages (AMP) versions of content, which is a different purpose than specifying a canonical URL.