Today it has a broader meaning, but rarely refers to the keywords meta tag. Recent examples of Keyword stuffing has become more content focused, and it still does work when done properly – but most blackhat SEOs all make the same mistake, that is to overdo it, and hence trigger penalties in the SERPs.
Avoid keyword stuffing
More recent examples would include web pages trying to rank for any given Keywords or Keyphrases – the silliest of attempts would be very obvious, in that they simply throw in a list at the bottom of the page somewhere, where they mention incoming search keywords and phrases.
The search phrases and keywords that other people used to find a given page on your website, is rarely of any relevance to your users – the only reason why someone would attempt to include them, is to archive better rankings for those phrases and words.
There have also been cases where websites would include long lists of keywords, in a very small font-size, somewhere towards the bottom of the page – this is less obvious, but it is also near worthless – and it clutters the page and its source with useless junk.
Use of strong and em
Some of the really poor techniques that some blackhatters use, is abusing strong and em places that they do not belong – and the strangest part is that they do this believing it will improve their rankings, while at the same time their rankings is just dropping.
Strong and em does likely have a very small significance, but abusing it can trigger penalties – these are not always obvious, as penalties placed on a site or page, will not always mean that its completely de-indexed. Strong and em are not the only offenders however, the continuous miss-use of headings has also been the source of problems.
Headings on a Page
Stuffing the headings with keywords is a little harder, and it is also a lot easier to detect – not doing it is likely a good idea, unless you want your site to rank very poorly. Something else that some blackhatters will attempt, is to use h1 for all their headings – while it will be unlikely to course any damage in itself, it may contribute to existing penalties, making the site rank even lower in the results.
Generally speaking, what a site is able to get away with, can largely be attributed to its authority rank – this is, unlike pagerank, not known to the public. There are many sites who is doing worse things then using h1s for all their headings, and they are still outranking other sites – another thing to consider, is that h1 may be used as the only heading in the new section system of HTML.