Wed Jan 09 20:02:59 PST 2019

Links in submission-required forms
Forms can include something as basic as a drop down menu or as
complex as a full-blown survey
In either case, search spiders will not
be accessible via a form are invisible to the engines

Links in un-parseable javascript
If you use Javascript for links, you may find that search engines
either do not crawl or give very little weight to the links embedded
within
Standard HTML links should replace Javascript (or
Links pointing to pages blocked by the meta robots
tag or robots
txt
The Meta Robots tag and the Robots
txt file (full description here)
both allow a site owner to restrict spider access to a page
Just be
warned that many a webmaster has unintentionally used these
directives as an attempt to block access by rogue bots, only to
discover that search engines cease their crawl

Links in frames or I-frames
Technically, links in both frames and I-Frames are crawlable, but
both present structural issues for the engines in terms of
good technical understanding of how search engines index and
Links only accessible through search
a common problem that it bears mentioning
Spiders will not attempt
millions of pages are hidden behind completely inaccessible walls,
doomed to anonymity until a spidered page links to it

Links in flash, java, or other plug-ins
The links embedded inside the Panda site (from our above example)
is a perfect illustration of this phenomenon
Although dozens of
padnas are listed and linked to on the Panda page, no spider can
to the engines (and un-retrievable by searchers performing a query)

Links on pages with many hundreds or thousands of
links
Search engines tend to only crawl about 100 links on any given page

This loose restriction is necessary to keep down on spam and
conserve rankings

seomoz
org rel="nofollow">Lousy Punks!
Links can have lots of attributes applied to them, but the engines ignore nearly all of these, with
to help stop automated blog comment, guestbook, and link injection spam (read more about the
launch here), but has morphed over time into a way of telling the engines to discount any link
value that would ordinarily be passed
Links tagged with nofollow are interpreted slightly
differently by each of the engines
You can read more about the affect of this and PageRank
sculpting on this blog post

can be used with the following syntax:
Google
nofollowed links carry no weight or
impact and are interpreted as HTML
text (as though the link did not exist)

they will not count those links in their
link graph of the web at all

Yahoo! & Bing
Both of these engines say that
nofollowed links do not impact search
results or rankings, but may be used by
their crawlers as a way to discover
new pages
That is to say that while
not count them as a method for
positively impacting rankings

Ask
com
Ask is unique in its position, claiming
that nofollowed links will not be treated
any differently than any other kind of
algorithms (based on local, rather than
global popularity) are already immune
to most of the problems that nofollow is
intended to solve