When Google crawls your site, ideally you want it to be able to see your site as a regular users would. If linked image, CSS, or JavaScript files are being blocked from crawling then Google is not able to properly render your site.
The problem with that is you’d likely never know when those files are being blocked and causing issues for your website. Google has announced that starting today, Webmaster Tools will include a Blocked Resources Report to help identify these types of issues.
The report is designed to first show you where the blocked resources are being hosted, then clicking on the host will reveal a drop down list of all the blocked resources along with the pages where they’re embedded. The report will also guide you through the steps you need to take to resolve the issues.
Curious why these files are so important? The updated Fetch and Render tool shows you why they matter. When you submit a URL to the tool it will show you how the page looks to a regular user compared to how it looks to Googlebot. If you have any blocked resources on your site, you will quickly see why they are so problematic.
If you have an abundance of blocked resources, Google recommends starting with the ones that will create the greatest visual difference when they become unblocked. Also keep in mind that in the blocked resources report, Google will only show you files from hosts that it believes you have some control over — meaning you won’t see files from hosts used by many different sites.
For more information on blocked resources, see Google’s Help Center article.
