But not all. And only those that occur in the visible text of the document. As far as Yandex considers keywords – check hard. Very difficult to find two equally relevant search query (that is best suited to this search page), containing a different set of keywords. However, one can exactly say that if your page does not contain words that you wrote down the tag keywords, then on the search, no Russian-language search engine finds your page. And there is nothing to try and reinvent the unimaginable set of keywords. If the words 'Abstracts' not in the text of your page, then search engines do not care if you tag keywords stuffed hundreds of words 'abstracts' (this is the most popular in RuNet search query). Your page will not be found even in last place search results (not that that would be somewhere in between).
Let's try it yourself to check (in general, any statement creators of the search engines must be checked. They are sly, sometimes simply powdering our brains that we do not know their secrets). Here is an example. Take the first available site Immigration to America (he caught me on the second page of the section 'Work' ranking Rambler's Top 100, quite by accident). Let's see the html-code main page of this site (to see the html-code page opened in a browser, users should click the right mouse button anywhere on the page and from the dropdown menu, select 'View Source' – this is for IE6, for other browsers are looking for such a reference, pop-up window text editor we are ready to see the html-code of our pages).