SEO Mistakes & Best Practices

- If a link points to inaccessible content (i.e., blocked by the robots.txt or a 404), it is indexed without related content.
- Unnatural links don't pass PageRank.
- Algorithms don't search for unnatural links 'per industry' because they would have a bad reputation, all industries are treated the same way.
- Some sites may rank despite bad backlinks because of other strong factors.
- Google is proactive in the detection of negative SEO performed with backlinks, it has algorithms detecting such practices.
- A lot of links (eventually auto-generated) between sites belonging by the same owner or group of owners can be considered a link scheme.
- URLs with a trailing slash and without are considered as different URLs.
- URLs are case-sensitive
- Socially shared URLs are taken into account by Bing and Google, if the corresponding page is public.
- There is no limit to the number of inner links in a page.
- If there is proper content edition, then the links in the content are considered natural.
- If a webmaster is informed and aware there is a link in the widget/badge he is adding to his site, then there is no risk of being accused of link scheme building.
- Offering users to set a link to NOFOLLOW in a widget/badge is a good practice.
- Off topic links in widget/badges are not ok.
- Naked links (i.e., text with no anchor) are extracted and treated like NOFOLLOW links, they are crawled for content discovery.
- There is no such thing as keyword stuffing in URLs.
- If a page contains two links, one DOFOLLOW and one NOFOLLOW, the PageRank will pass.
- If it is a requirement or an obligation to have a DOFOLLOW link in some hosted content, then such links are considered unnatural.
- For better user experience, it is recommended to implement a 'Resource not found' page rather than an empty 404 error page.
- PageRank does not pass through 404's an 410's, the content is completely ignored.
- A lot of 404's on a website is a bad signal regarding quality.
- When a soft 404 is detected it is treated like a 404.
- NOFOLLOW links don't pass any signal, but they may be crawled unless they are blocked by robots.txt.
- Algorithms detect when websites link to everything, and ignores them.
- Javascript links bear PageRank.
- Great websites without backlinks may not be detected/ranking by Google.
- Forwarding a domain forwards its signal too.
- Forwaring a spammy website with 301 for negative SEO is well detected and neutralized by Google.
- Big websites are less impacted by negative SEO attacks than small. websites
- Google attempts to detect all irrelevant and unnatural links algorithmically.
- If they are certain they capture unnatural links, Google algorithms may demote a website a little.
- If there are many unnatural outbound backlinks, Google may ignore all links on that site.
- Ideally, content should be so great that people would naturally link to it.
- A natural link is considered as a natural recommendation.
- As long as a link is not trusted, it does not count. PageRank and signals are not passed
- Link to 404 page are dropped after some time (signals are ignored)
- Inorganic link is something a webmaster did not place on their own or link that were paid for
- Pages with faulty redirects are demoted.
- NOFOLLOW are dropped from the link graph.
- Canonical links don't work for images or videos.
- Disavow links even if they are NOFOLLOW, because they could become {{DOFOLLOW} in the future.
- Links are used as recommendations.
- Tries to take all links into account (even those not displayed in Google Search Console).

About Canonical Links & SEO Redirects

- Google needs to trust the canonical links to take them into account.
- Canonical links can be ignored if the source and target pages have very different content or if there is evidence the webmaster made a mistake (for example pointing to an unexisting page).
- Canonical links combine the signals from different pages into the target URL page, including PageRank.
- Essentially, canonical links are treated like 301s.
- Be consistant with canonicals, avoid circular references.
- A 301 redirect has the priority over canonical links.
- A canonical link has the priority over hrefang.
- Google classifies all redirection codes 3xx into temporary or permanent.
- A large number of 301 redirections on a website is not a negative signal for rankings.
- Avoid cascading 301's, try to point to the end page directly.
- Do pair canonical and hreflang properly.

You can report people selling or using paid links here.