According to Wikipedia1, there are some rules some spiders follow:
<!--googleoff: all-->
This should not be indexed by Google. Though its main spider, Googlebot,
might ignore that hint.
<!--googleon: all-->
<div class="robots-nocontent">Yahoo bots won't index this.</div>
<noindex>Yandex bots ignore this text.</noindex>
<!--noindex-->They will ignore this, too.<!--/noindex-->
Unfortunately, they could not agree on a single standard it seems – and to my knowledge, there's nothing to keep all spiders off...
The googleoff:
comment seems to support different options, though I'm not sure where there's a complete list. There's at least:
- all: completely ignore the block
- index: content doesn't go into Google's index
- anchor: anchor text for links will not be associated with the target page
- snippet: text will not be used to create snippets for search results
Note as well that (at least for Google) this will only affect the search index, not the page ranking etc. Furthermore, as Stephen Ostermiller correctly pointed out in his comment below, googleon
and googleoff
only work with the Google search appliance and have no effect on normal Googlebot, unfortunately.
There's also an article on the Yahoo part2 (and an article describing that Yandex also honors <noindex>
6). On the googleoff:
part, also see this answer, and the article I took most of the related information from.3
Additionally, Google Webmaster Tools recommend using the rel=nofollow
attribute4 for specific links (e.g. ads or links to pages not accessible/useful to the bots, such as login/signup). That means, the HTML a rel Attribute should be honored by the Google bots – though that's mainly related to page rank, not to the search index itself. Unfortunately, it seems there's no rel=noindex
5,7. I'm also not sure if this attribute could be used for other elements as well (e.g. <DIV REL="noindex">
); but unless crawlers honor "noindex", that wouldn't make sense either.
Further references:
1 Wikipedia: Noindex
2 Which Sections of Your Web Pages Might Search Engines Ignore?
3 Tell Google to Not Index Certain Parts of Your Page
4 Use rel="nofollow" for specific links
5 Is it a good idea to use <a href=“http://name.com” rel=“noindex, nofollow”>name</a>
?
6 Using HTML tags — Yandex.Help. Webmaster
7 existing REL values
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