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Making something bold makes it more important for search engine ranking and even improves readability for humans.
To get the search engine ranking benefit, do we need to use <b> or <strong> tag or are CCS rules to do this recognized?
In other words, do search engine crawlers actually read and apply CSS when analyzing content?
The generally accepted way to emphasise text is to use the <strong> tag. You can then style that using CSS.
Looking through our logs I can see some evidence that some spiders fetch CSS files. Specifically,
Anecdotally there's evidence that some search engines use this to detect if text is being hidden (and therefore someone is trying to fool a search engine with bogus keywords).
I think your safest bet is to use <strong> and don't rely purely on CSS to emphasise important words or phrases.
