Most of these recommendations <i>are</i> based on measurements.  I recall a coworker doing several experiments on Google Search about inlining CSS vs. making a separate request, and the break-even point was surprisingly high (~10K or so of CSS).<p>Publishing recommendations like this is a recognition that many small companies don&#x27;t have the resources that Google does to rigorously experiment with different approaches and collect data.<p>That said, if you <i>do</i> have the resources, you should always measure &amp; experiment with your own situation.  Many of these recommendations go away with HTTP2, for example, and it&#x27;s likely that the guidelines won&#x27;t be updated until long after HTTP2 is widely adopted.