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rel= n'est pas une balise mais un attribut d'une balise
Pour savoir à quoi ça sert il faut s'intéresser aux balises qui utilisent rel= comme attribut.
C'est comme l'attribut height=, il peut s'utiliser dans différentes balises et il n'aura pas toujours le même sens en fonction de la balise à laquelle il est associé
alternate : Links to an alternate version of the document (i.e. print page, translated or mirror)
author : Links to the author of the document
bookmark : Permanent URL used for bookmarking
help : Links to a help document
license : Links to copyright information for the document
next : The next document in a selection
nofollow : Links to an unendorsed document, like a paid link.
("nofollow" is used by Google, to specify that the Google search spider should not follow that link)
noreferrer : Specifies that the browser should not send a HTTP referer header if the user follows the hyperlink
prefetch : Specifies that the target document should be cached
prev : The previous document in a selection
search : Links to a search tool for the document
tag : A tag (keyword) for the current document
rel=”noreferrer noopener”
This tag when combined basically means that no referrer information should be passed to the website being linked to and prevents the newly opened page from having the ability to control the page that delivered the traffic.
There is something called a http “referrer header” in your website code that adds information about your visitor like browser, device, location etc. to pass through to the page that visitor is clicking through to. This new tag prohibits that information from being passed to the referral page.