To me, the fact that a nearly $1 trillion dollar company has decided to no longer develop their own web browser engine is a sign that the HTML specification has become too complex to properly implement.
But rather than review the HTML specification (and perhaps provide more detailed implementation hints for creating your own HTML browser), instead, we move to a world where the de facto specification is not the de jure specification, but whatever is implemented in Chrome, which is itself based on WebKit–derived from the KDE HTML layout engine.
I’m always concerned when specifications become too complex for implementation by mere mortals.
It also means certain aspects of the HTML specification–such as elements of the HTML specification used by ePUB (such as paged layout) is highly dependent on either undocumented API hooks inside a massive and hard-to-understand third party library, or is simply impractical to implement.
And it suggests to me that any technology that decides to rely on the HTML specification for something like page layout automatically limits the implementation of that technology. For example, it makes creating an ePUB reader that isn’t essentially a full Linux installation with a web browser launched at startup time nearly impossible–and that means there will be a lot of really crappy and horrendously insecure ePUB readers out there.