How do the `rel` attributes in HTML affect SEO?

The attribute rel in HTML elements, particularly within the tags <a> and link, can have a significant impact on search engine optimization (SEO). Various values of rel directly influence how search engines understand and treat links. Below are the values of rel most relevant that affect SEO:

canonical

• Description: Indicates the preferred version of a page when duplicate versions exist.
• Usage: Helps search engines understand which version of a page to index and position, avoiding duplicate content issues.
• Example:
  html

nofollow

• Description: Indicates to search engines not to follow the link or pass «PageRank» (or its equivalent) to the linked page.
• Usage: Useful for user-generated content, paid links, or any link you do not fully trust or endorse.
• Example:
  html <a href="https://external-site.com" rel="nofollow">External Site</a>

noopener

• Description: Prevents the new page from accessing the window object of the source page, enhancing security. Although not directly an SEO attribute, it is often used in conjunction with noreferrer.
• Example:
  html <a href="https://external-site.com" rel="noopener">External Site</a>

noreferrer

• Description: Prevents the browser from sending the referrer URL to the new page. Implicitly includes the behavior of noopener in modern browsers.
• Usage: Although its primary purpose is privacy, it can also affect how traffic sources are reported, which can be relevant to SEO.
• Example:
  html <a href="https://external-site.com" rel="noreferrer">External Site</a>

prev and next

• Description: Used to indicate the relationship between component URLs in a paginated series.
• Usage: Helps search engines understand the structure of paginated content.
• Example:
  html

alternate

• Description: Indicates alternate versions of content, such as mobile versions or translations.
• Usage: Useful for indicating mobile versions or variants in different languages of the same content.
• Example:
  html

external

• Description: Indicates that the linked content is external to the current site.
• Usage: Its impact on SEO is not explicitly documented, but it can be used for semantic clarity.

ugc (User-Generated Content)

• Description: Introduced by Google, this value indicates links within user-generated content, such as comments and forums.
• Usage: Ayuda a Google a identificar y tratar los enlaces que no respaldas editorialmente.
• Example:
  html <a href="https://external-site.com" rel="ugc">User comment link</a>

Indicates that the link is an advertisement, sponsorship, or other compensation agreement.

• Description: Indica que el enlace es un anuncio, patrocinio u otro acuerdo de compensación.
• Usage: Indica a los motores de búsqueda que no consideren este enlace para propósitos de posicionamiento.
• Example:
  html <a href="https://external-site.com" rel="sponsored">Advertising Link</a>