What is CSS ?
Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language.
CSS and Semantic Markup:
CSS is commonly mentioned as a best practice for general web design and development, but its principles provide some indirect SEO benefits as well. Google used to recommend keeping pages smaller than 101 KB, and it used to be a common belief that there were benefits to implementing pages that were small in size. Now, however, search engines deny that code size is a factor at all, unless it is extreme. Still, keeping file size low means faster load times, lower abandonment rates, and a higher probability of being fully read and more frequently linked to. This is particularly important in mobile environments.
It also used to be that the search engines could not read CSS code and render pages in the same manner as a browser does. Google made it clear in October 2014 that it is able to do that (http://googlewebmastercentral.blogspot.com/2014/10/updating-our-technical-webmaster.html), so good clean page layout as setup by your CSS could potentially be considered as a factor in evaluating page quality.
You can use CSS code to provide emphasis, to quote/reference, and to reduce the use of tables and other bloated HTML mechanisms for formatting, which can make a positive difference in your SEO. Be sure to allow Googlebot access to your CSS files.
Google, Bing, and Yahoo have come together to create a standard for markup called Schema.org (http://www.schema.org). You can see a copy of the announcement at this URL” http://googleblog.blogspot.com/2011/06/introducing-schemaorg-search-engines.html. This represented a new level of commitment by the search engines to the concept of marking up content, or more broadly, in allowing the publisher to provide information about the content to the search engines. When we refer to marking up content we refer to the concept of tagging your content using XML tags that categorize the contents of a block of content. For example, you may label a block of content as containing a recipe, and another block of content as containing a review.
This notion of advanced markup was not new, as all of the search engines have supported semantic markup at a limited level, and have used this markup to show rich snippets.
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