An update to Google technical webmaster guidelines

As our crawling and indexing systems improved to render pages as part of normal indexing, today we're updating our webmaster guidelines to reflect that. Our new guidelines explain you should allow Googlebot to crawl the page assets (CSS, JavaScript, etc) so that we can index your content properly.

Let me be super clear about what this means: By blocking crawling of CSS and JS, you're actively harming the indexing of your pages.  It's the easiest SEO you can do today. And don't forget your mobile site either!

Read more in the announcement:

and also our announcement earlier this year about using rendering as part of indexing:
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