While these points are valid, and indeed very insightful in some cases (the logging / encryption issue hadn't even occurred to me before now), I do wonder if the author would ever be happy. Using bleeding edge features will always cause cross-browser problems like this, no matter what year you're in, no matter which browsers are currently popular, no matter how useful the features are. The other way to look at it is that every single year, there are more things you can safely use than the year before. In 5 years time when 95% of your users support all of the features he mentions, will he be rejoicing, or complaining that HTML6 support is spotty?
Also worth mentioning that for some of the issues (like differences in cross-domain feature detection) they should really be handled by libraries to abstract the differences between underlying browsers from the point of view of your business code. Browsers will always have differences, fix the problem once and abstract it away!
> Using bleeding edge features will always cause cross-browser problems like this
Well, the problem is that these aren't bleeding edge features. FileSystem API is the most bleeding of all mentioned and that's been in stable Chrome for almost a year. The others have been available in some combination of stable browsers for multiple years. (And most were specced 3+ years ago)
Compare this sort of behavior with something like Windows Metro, iOS, Android, where apps are using newly introduced APIs within weeks of release, and I think he's justified in wanting better compatibility out of the web platform.
Also worth mentioning that for some of the issues (like differences in cross-domain feature detection) they should really be handled by libraries to abstract the differences between underlying browsers from the point of view of your business code. Browsers will always have differences, fix the problem once and abstract it away!