Google has come a long way with javascript email list . And as various javascript frameworks are increasingly used to publish content, it is extremely important to ensure that google can accurately render all content on a page. If for some reason google cannot accurately display your pages, some of this content may not be indexed. And in the worst case, significant parts of your main content will not be indexed. First off, if you're unfamiliar with how google renders javascript, then this video segment by google's martin splitt sums it up nicely (at 10:46 in the video).
Google crawls the page and sends the page to the processor for rendering. Static html is indexed while found links are sent back to the crawler for discovery. The email list processor connects the page to chrome (version 41 onwards) and renders content, including content posted via javascript. Then the resulting page is indexed, while any additional links found are sent back to the crawler for discovery. So there is a two-phase approach to indexing javascript content:beyond content discrepancies in the rendered html, you may find that important directives are missing or have been moved to a new location in the code.
I recently wrote a case study where the email list robots meta tag moved a thousand lines in the code, out of the head and into the body of the document. Scary, right? For site owners and seos, this is why it's extremely important to test your pages to ensure that all of your important content is rendering correctly. Unfortunately, while helping many companies over the past few years, I've noticed that a number of them don't know the best ways to display rendered html. They just looked at their pages in the current version of chrome, maybe explored their sites (without javascript rendering) and thought everything was fine. Unfortunately, this leaves a lot of room for error. And again, in the worst case, a site can end up with a significant portion of the content not rendered. So testing is really, really important.