<a> tags can have the rel attribute applied to outbound links, that helps browsers and search engines understand the relationship your website has with the link.
The three most relevant values are:
- rel="nofollow" Tells search engines that they should not follow this link when crawling
- rel="sponsored" Marks as advertisement, paid backlinks, or other monetary gain
- rel="ugc" Marks the link as generated by a user (not the owner) of the website, commonly used for links in comment on blog posts, forums etc.
Generally speaking, you would mark links to external sites you don't fully endorse with nofollow, links where you stand to gain monetarily as sponsored, and have all links by users as ugc.
Links to sites that you fully trust, but don't gain money for linking to, are free to be without a rel attribute.
Note: You can apply multiple values to the rel attribute, such as rel="ugc nofollow".
Note: The attributes noopener and noreferrer can be used to prevent the website you link to from knowing where the link came from.