Cascading Style Sheets (CSS) are widely recognized for their contributions to building fast-loading, standards-compliant, easily modifiable web pages. In addition to these well-known capabilities, CSS can provide some compelling SEO/SEM benefits. These include:

Making pages "lighter" in weight, and decreasing the code-to-content ratio. Done properly, this can make your entire site easier and more efficient to crawl, leading to more and better indexing of your site's content.

Using standard HTML tags, such as <h1>, which search engines recognize as indicators of important content, instead of fancy <font> tags

Using standard HTML tags, such as <ul> and <a> to construct menus that contain standard HTML links to other important areas of your site. This makes these links easily discoverable by search engines, helping ensure more efficient and deeper crawling of your site.

Moving important content to the top of the HTML file. In the past this was the single most important benefit that CSS could bring to SEO, but that is no longer the case. Placement of the content high in the HTML used to potentially affect the importance that search engines attached to the content, but now it is more about helping ease discoverability.

CSS can also provide these benefits in your ongoing search engine optimization and search engine marketing efforts:

By separating content and presentation, you can very easily experiment with different copy and presentation (for example font, background color, etc.) and test what converts best
Less time spent maintaining web sites frees up webmaster resources, helping you to implement more and fresher content, and more SEO/SEM strategies.

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