Many organisations (especially universities) have satisfactory support systems for those who create or manage web content. Indeed, some are superior to the support for 'ordinary' work writers. And there's the rub: online writing is separated out as special, unusual, not part of mainstream business writing at all.
Online writers are identified as part of a specialised web community. But I doubt they see themselves that way, because most everyday writers are not technically minded. They are simply people who write stuff for work.
The struggle is to convince staff that everyday business writing is, de facto, web writing. Certainly, content writers need to be wooed and won, but not in a way that implicitly differentiates web writing from other business writing. After all, the principles of web content writing (concise, skim-readable, objective) are valuable for nearly all business writing.
In a Contented organisation, a majority of staff would identify themselves as potential writers of online content, and so they web-proof their documents in advance. It happens every day: you write a report never dreaming it will be put on the intranet then ouch, your boss decides to do just that. There it is, either looking ridiculous because it wasn't written for the intranet or perfectly OK, because you followed a few simple principles.
Traditionally, all guidelines for web sites and intranets have been combined in one predominantly technical web style guide. That is still necessary for the team that manages and maintains sites. But content writing is more likely to improve if the regular company style guide is founded on guidelines for web content, and not just as an add-on. Web style should be the default style. After all, if every document starts with a clear descriptive headline and summary, is skim-readable and written in plain language, then the document is both well written and virtually web-proofed.
Trapping web content guidelines in a glasshouse will only help individual writers of new web content. It cannot improve the print-based documents that often need to be uploaded, nor raise the general standard of business writing.