The keywords meta tag, included in the metadata of an HTML document, has traditionally provided page content information to search engines. Back in the 90s and into the early 00s the keywords meta tag was used heavily by search engines to rank pages. Like anything that’s open to abuse it was exploited by web workers keen to gain search engine favour by stuffing the tag with anything they could dream up. As a result Search engines started to turn their back on keywords and develop more complex ways of ranking pages, the rest is history.
<meta name="keywords" content="HTML, CSS, XML" />
It’s a shame this happened, keywords would almost certainly have been of some use somewhere in the scheme of what we’re currently doing with the web. Saying that, It’s not necessarily recommended that the tag being removed, just that Google won’t be recognising it any more. But if Google aren’t looking at something behind the scenes then there’s not really much reason for using it. Especially if you’re pulling the content from a CMS and taking up valuable rendering time with variables and manipulation.
If there’s any doubt in the deprecation of keywords, here’s a video featuring Google engineer Matt Cutts talking about the subject way back in 2009.