As someone who was around the WHATWG from relatively on, and worked at Opera over the period when it moved to Chromium, I'd strongly suggest that HTML5 was a _massive_ win for Opera: instead of spending countless person hours reverse-engineering other browsers to be compatible with existing web content, things like HTML parsing become "implement what the spec says" (which also implicitly made it match other browsers). Many of the new features in the HTML5 spec of that period were things that either were already basically supported (in SVG especially) or were relatively minor additions; I don't think they played a significant role either.
There's a good reason why Opera was heavily invested in the WHATWG, and it's the fact that by having a spec implemented cross-browser defining how to parse HTML and how cross-page navigation works you eliminate one of the biggest competitive advantages more major browsers have: web compatibility, both through legacy content targetting the browser specifically and also web developers being more likely to test in those browsers. (And to be clear—this would've been true for any form of error handling, including XML-style draconian error handling; but the long-tail of existing content needed to be supported, so even if major sites had migrated to XML you still needed to define how to handle the legacy content.)
The downfall of Presto is arguably mostly one of mismanagement and wrong priorities decisions being made; I don't think Presto was unsaveable, and I don't think the Presto team was so significantly under-resourced it couldn't compete.