The problem you describe does exist but I respectfully disagree that's the reason why writing web applications has become as cumbersome as it has. I think the issue there is more down to standards authorities being glacially slow to recognise the change in demands. This has obviously allowed a situation where browser vendors such as Microsoft and Google have felt they needed to run amok just to offer many of the features developers were asking for (and to an extent, consumers too since end users were lured in by prettier and more interactive sites).

There have been other examples in history where programming languages used to differ - sometimes even significantly - depending on which compiler / platform you were targeting and where a standards body later stepped in to create a basic subset of said language that should be universal across all dialects (please note they cannot enforce this). In those instances that has lead to code to become greatly more portable.

To some extent, this is now happening with the web as well; however my secondary point to the complaint about differing outputs between browsers is that I believe HTML et al is a lousy way to design applications from the outset. That definitely is not a problem created by warring proprietary vendors or slow revisions of standards but rather just an artefact of technology evolving past it's original purpose yet still having to retain backwards compatibility. Maybe the time has come that we need a second language for the web so we have HTML et al for legacy applications, blogs and other stuff that is following some of the original visions of the web, but have a new language for web applications and anything that requires a stronger security model.