Since the early days of rendering JavaScript, Google has come quite a long way, and because of all the different JavaScript frameworks being used more and more to publish content, it’s critically important to make sure Google is able to accurately render all of the content on a page. If it ends up that Google isn’t able to accurately render your pages, then some of that content might not e ale to be indexed.
Here is a video segment from Google’s Martin Splitt that helps to sum up how Google renders JavaScript (if you’re not familiar with it already). It begins at 10:46 in the video. Google crawls the page and send the page for rendering. The static HTML is then indexed while any links that are found are passed back to the crawler for discovery. The processor plugs the page into Chrome (version 41 as of now) and renders the content, including content published via JavaScript. Finally, the resulting page is then indexed, while additional links that are found are passed back to the crawler for discovery.
For site owners and SEOs, it’s very important to test your pages to make sure all of your important content is being rendered properly. A recent case study was written saying that he meta robots tag was moving on thousand lines down the code, and out of the head and into the body of the document. That’s pretty scary. It seems that a number of companies didn’t know the best ways to view the rendered HTML. They took a look at their pages in the current version of Chrome, possibly crawled their sites (without JavaScript on) and assumed that everything was good. This ends up leaving plenty of room for error In a worst-case scenario, it’s possible that a site can end up having big chunks of content not being rendered at all.
This is why rendering is very important.
Glenn Gabe wrote a post that covers six different was you can view the rendered HTML for your pages. The focus will be square on Google’s own tools, but some third-party tools will be looked at as well.