The Meta Keywords tag was originally designed to be used to highlight keywords or keyterms (search terms) targeted by the page, to inform search engines what terms the page should rank for. Previously, search engine algorithms were not able to identify the relevance of these terms for themselves, hence webmasters had to manually highlight them using the Meta Keywords tag. As these have previously used for spam purposes (excessively highlighting keywords or including terms not featured on the page) meta keywords are no longer used by most SEO professionals. Many rookie SEO professionals and web developers attempting SEO still use Meta Keywords tags, contrary to their redundant nature.
Search engine interpretation of Meta Keywords
Google has never used the Meta Keywords tag, and all major search engines including Bing as well as other major search engines around the world, such as Yandex (Russia’s main search engine) and Baidu (China’s main search engine), have long since followed in Google’s footsteps in ignoring meta keywords.
A little known fact is that the greatest SEO advantage actually comes from not using Meta Keywords tag. This is because the code that would be needed will increase page loading times by a fraction of a second, which in turns makes for a very slightly poorer user experience (more so to people on extremely slow or intermittently working internet connections, or people who pay per data). This slight decrease in user satisfaction, however slight it may be, is still greater than the SEO benefit of using meta keywords (which would be making your site compatible with extremely insignificant search engines). Many SEO experts also suspect that Google looks out for spammy meta keywords as an indicator that a site has spammy intentions. While this is unlikely to be built into their algorithm, it may hold some water in extreme cases when a human member of Google’s staff conducts a manual spam review of your site.
Redundancy is different for Meta Description tags
Note also, that Google at one point shunned the use of Meta Description tags for ranking purposes, stating that they are only used for display in search results which may impact on click-through rate (CTR), but like the Meta Keywords tag they do not affect ranking performance. Google has since taken back this statement, saying that they may affect ranking performance, and logic dictates that if the Meta Description increases CTR for irrelevant traffic that will cause a high bounce rate, which is definitely being measured by Google as one of the main ranking factors these days. Therefore Meta Keywords tags are safely considered redundant, but Meta Description tags should be considered more carefully.