Issue link: https://read.uberflip.com/i/1146286
YouTube | 137 In the beginning of the page load, the main hurdle to parallelization is the six scripts downloaded back-to-back. As explained in Chapter 6, scripts block all other down- loads, no matter what their hostnames are. Additionally, the scripts aren't minified. Combining these six scripts into a single script and minifying them would decrease the download time. Also, if any of these scripts could be downloaded later in the page, the initial part of the page would be downloaded and rendered sooner. At the end of the page, decreased parallelization results from downloading 15 images from a single hostname (img.youtube.com). YouTube only uses four unique host- names in their page. It would be worth the cost of an extra DNS lookup to split these 15 downloads across two hostnames and double the amount of simultaneous down- loads. Sadly, not a single component has a far future Expires header (Rule 3). Most of the components in the page are user-generated images that rotate frequently. Adding an Expires header to these might have little benefit, but the other components in the page don't change so often. Eleven of the components haven't changed in six months or more. Adding a far future Expires header to these components would improve the response times for subsequent page views. YouTube uses the Apache web server, and their components still contain Etags, but YouTube has made the extra effort to modify the ETag syntax to improve their cacheability, as explained in Chapter 13.