1. Extremely powerful backlinks come from relevant, standard websites
The quality of the backlink source website is a key factor in determining its importance.
A single backlink to an authoritative and relevant website is much less valuable than backlinks from low quality websites.
Here are some key metrics for estimating the quality (or strength) of backlinks based on your source website and some key HTML attributes.
Domain Authority - A website's domain authority metric is a measure (on a scale of 0 to 100) of how popular a website is in its power or industry. The higher the domain authority of a website, the more valuable it is to search engines. A backlink from a high domain authority website will generally be so powerful that it will have a positive effect on your search engine rankings.
Reference flow - The reference flow metric of a website is between 0 and 100 and indicates the number of backlinks that point to it. If a website has a large number of links to a website, then a website will have a high referral flow. Backlinks from a site with a great website can fetch a good price.
Trust Flow - A website's trust flow metric, again, scores between 0 and 100 and indicates the quality of the backlinks. If several standard websites are linked to it, a website will have a high trust flow. Similarly, if it has backlinks from sketched websites, its trust flow score will drop. A backlink to a website with high trust flow metrics will usually be valuable.
Spam Score - A website's spam score metric is a measure (on a scale of 0 to 17) that indicates the likelihood that the website will be penalized by search engines (such as being spammy). A spam score of less than five can be considered safe. A backlink to a website with a spam score under the age of five will be considered a good backlink.
Doflow - An effective backlink is always a do-follow backlink (described in detail in the following section).
Index Status - Indicates the index status of a web page whether or not it has been indexed by search engines. An unsorted webpage may not appear in search results. A website (or webpage) often has non-indexing status if it is penalized by Google or if its robot.tst file is not properly formatted. A backlink to a page that is not indexed does not give you any SEO juice. In addition, if such a backlink is from a website that has been wiped out of Google's index as a result of a fine, it can also damage your website's ranking.
External link count - A page's external link count metric is the number of external links on a page. If a web page links to tons of external links, you'll want to check if the page is spam. A backlink from a website (or a web page) that links to multiple pages may not be as valuable.