This book breaks downs a few very important points about HTML5 that other books I’ve read on the topic do not, and they all involve history. Jeremy Keith explains how we got here, from the beginning with HTML 2.0 through the WHATWG and WC3 kerfuffles to the present (2010) day.
Knowing the history of HTML helps considerably in understanding what decisions were made and why. Understanding the design principles — especially in the light of graceful degradation — is also quite helpful.
I read both the first edition and the second edition. The bulk of the second edition changes are dedicated to updating the features and options that have become browser-supported since the first edition came out in 2010. Because of the solid principles Jeremy Keith describes the web standards groups are using, the first edition isn’t inaccurate so much as missing all the cool stuff that’s developed since so if this is the only edition you have access to, it’s still worth the read.