If resources like JavaScript or CSS in separate files are blocked (say, with robots.txt) so that Googlebot can’t retrieve them, our indexing systems won’t be able to see your site like an average user. We recommend allowing Googlebot to retrieve JavaScript and CSS so that your content can be indexed better. This is especially important for mobile websites, where external resources like CSS and JavaScript help our algorithms understand that the pages are optimized for mobile.

If your web server is unable to handle the volume of crawl requests for resources, it may have a negative impact on our capability to render your pages. If you’d like to ensure that your pages can be rendered by Google, make sure your servers are able to handle crawl requests for resources.

It's always a good idea to have your site degrade gracefully. This will help users enjoy your content even if their browser doesn't have compatible JavaScript implementations. It will also help visitors with JavaScript disabled or off, as well as search engines that can't execute JavaScript yet.

Sometimes the JavaScript may be too complex or arcane for us to execute, in which case we can’t render the page fully and accurately.

Some JavaScript removes content from the page rather than adding, which prevents us from indexing the content.

Posted by Erik Hendriks and Michael Xu, Software Engineers, and Kazushi Nagayama , Webmaster Trends Analyst

In 1998 when our servers were running in Susan Wojcicki ’s garage, we didn't really have to worry about JavaScript or CSS. They weren't used much, or, JavaScript was used to make page elements... blink! A lot has changed since then. The web is full of rich, dynamic, amazing websites that make heavy use of JavaScript. Today, we'll talk about our capability to render richer websites — meaning we see your content more like modern Web browsers, include the external resources, execute JavaScript and apply CSS.Traditionally, we were only looking at the raw textual content that we’d get in the HTTP response body and didn't really interpret what a typical browser running JavaScript would see. When pages that have valuable content rendered by JavaScript started showing up, we weren’t able to let searchers know about it, which is a sad outcome for both searchers and webmasters.In order to solve this problem, we decided to try to understand pages by executing JavaScript. It’s hard to do that at the scale of the current web, but we decided that it’s worth it. We have been gradually improving how we do this for some time. In the past few months, our indexing system has been rendering a substantial number of web pages more like an average user’s browser with JavaScript turned on.Sometimes things don't go perfectly during rendering, which may negatively impact search results for your site. Here are a few potential issues, and – where possible, – how you can help prevent them from occurring:To make things easier to debug, we're currently working on a tool for helping webmasters better understand how Google renders their site. We look forward to making it to available for you in the coming days in Webmaster Tools If you have any questions, please feel free to visit our help forum.