Nofollow Link and SEO
Introduced by Google in 2005, nofollow is a value that can be assigned to the ‘rel’ attribute of an HTML element ‘a’ to instruct search engines that the hyperlink should not affect the ranking of the link target of the search engine’s index. The usage of nofollow has an impact on link juice as none is passed on. It can also be used for an entire document.
<a href="https://www.example.com/" rel="nofollow">Example</a>
##for entire document <meta name="robots" content="nofollow" />
Why was it created and what is its usage
Google wanted to differentiate content links from other less valuable links such as in the comments, as a guest, or automated. It should be used for Purchased links/Affiliate links and paid advertising, Press releases, and Guest posts. Also it should be for any link that is not trustworthy. .
Nofollow and SEO
There has been so far no evidence that a too high proportion of nofollow links in a site’s backlink profile is a disadvantage although a benchmark can be established with successful competitors. A lot of important sites such as Wikipedia use exclusively nofollow links even for high profile and verified pages.
The alternatives (since 2019)
rel="sponsored" ##for sponsored links rel="ugc" ### for user generated content: comments and posts
How Search Engines treat nofollow
Google and Bing say they do not in general follow such links. Other such as Yahoo actually still follow such links but excludes it from the ranking calculations.