SEO people all pay attention to the normal links written in the <a href=http://www.360.gov.cn> tag.
There are also many unusual and hidden link forms on the Internet, and this post on Webmaster World lists many of them. This post is only two pages long, but there are many administrators and moderators showing up, which is rare.
Will these hidden links in the normal sense have an impact on search engines' discovery of the page, transfer weight, and page ranking? No one can say for sure.
For example, links that appear in gmail are tracked by Google. Click the link in gmail, and you will see the tracking code first appear in the address bar, and then redirect to the link in the real email. So far there is no indication that links in gmail will help page inclusion, let alone impact on rankings. But if Google never intended to exploit this data, why track it in the first place?
These unusual links include:
1 Other websites link to your images. Some websites will hotlink pictures, steal traffic, and may also give you a link.
2 Link to JS file.
3 Link to CSS file.
4 Link to RSS feed.
5 Links in emails that search engines can see, such as Yahoo mail, hotmail, and gmail.
6 Links with noindex.
7 Links with nofollow.
8 URL appearing in JS script or JS comments.
9 URLs that appear in CSS or CSS comments.
10 URLs appearing in metadata of pictures and video files.
11 The URL that appears in the comments of the HTML file. like:
12 URLs that appear in HTML file headers, meta data, and ALT tags. For example: <!-- http://www.rigeng.com -->
13 Links and pages visited by users that can be tracked by the toolbar.
14 The target URL can only be seen after executing the JS script.
15 Links in paid content. The webmaster can set it up through webmaster tools so that Google can crawl these links that can only be seen after logging in.
16. Links on domain names with 301 redirects.
17 Links in Flash.
18 URLs that do not contain plain text, such as http://www.265.gov.cn
19 Non-webpage files, such as world files, PDF, links in TXT files.
20 Use robots files to disable links on crawled pages.
21 Domain name registration data and DNS data.
22 Links that appear in web tables.
23 Links in other gadget software.
24 Links in non-traditional pages, such as Twitter, Yahoo, Google forums, etc.
25 Advertising links such as Google Adwords and links from other services, such as Google Maps.
26 Finally I would like to highlight the links that may appear in the image. We all know that search engines cannot understand images, which is why SEO people emphasize text content. But recently Google has made a lot of progress in image processing.
Google acquired an image processing company a few years ago, and recently it has been developing new tricks in areas such as identifying image color, type, and facial similarity. So can URLs be identified from images? If so, can advertising URLs that appear in Google Maps Street View photos be read? This is probably one of the directions Google is researching.