YouTube engineering manager Richard Leider explained why HTML5 was so valuable to YouTube in his announcement:
Combined with Common Encryption, we can support multiple content protection technologies on different platforms with a single set of assets, making YouTube play faster and smoother.
The most important feature that HTML5 will bring to YouTube is Google's VP9 video codec. Google claims that VP9 helps videos start 15 to 80% faster than other video codecs.
This isn't newfangled tech either, VP9 has been tested and used on other sites since 2013.
Want to test out Google's new HTML5 format? Click here to find out where to see official NFL highlights on YouTube.