We’ve already debunked the mysteries surrounding HTML5 itself — the pros, cons and progress — but one pesky question remains. Namely, what do Google and Apple, two companies that obviously thrive off sales and earnings, have to gain by backing a free-for-all open standard?

Neither can claim direct ownership over it, nor can they can they directly monetize it. Worst of all, HTML5 could pave the way for the dominance of web-based apps over natively-developed apps, a real concern for Apple, which, according to estimates by Gartner, could be booking as much as $4.5 billion in revenue from apps for 2010.

Despite that, or funnily enough, because of that, some analysts and developers believe Google and Apple will ultimately profit from HTML’s rise.

(See a gallery of the best HTML5 Apps out right now.)

Here’s how:

Google’s game: Cutting out enterprise competition

Google (GOOG) has been pretty up-front about its affinity for HTML5. In fact, YouTube, which Google acquired in 2006, already provides an excellent HTML5-driven web-based mobile app that some argue is better than its iOS counterpart. And this week, it’s expected the Chrome OS web store, which will sell HTML5-based Google apps, and a Google-branded Chrome OS netbook could see the light at All Things Digital’s D: Dive Into Mobile conference.

Like Apple, Google can’t make money directly off the web spec. But by supporting it, it increases the already great likelihood that software, once so predominantly built with proprietary tools for each and every platform, is instead built with web-based tools (read: HTML5). And as Google well knows, Internet apps are really the core of its business model: Gmail, Docs, Maps, and so on. Generally speaking, anything Internet-based gives Google the opportunity to do what it does well: provide more cloud-based products.

In order to make those products increasingly competitive, Google will inevitably need better browser performance and consistent browser performance across browsers. If functionality remains inconsistent among browsers, it will be much more difficult for Google to innovate. Establishing and working with HTML5 as the web standard means every browser will implement Google product features nearly the exact same way. Development will be quicker and more cost-efficient, and ultimately, Google builds an ecosystem of developers (and users) they can then automatically tie down to their apps.

“It’s a way to essentially take people out of the Microsoft (MSFT) ecosystem and even to some extent, out of the IBM (IBM) and Oracle (ORCL) ecosystem and bring them into their ecosystem,” says Forrester analyst Jeffrey Hammond.

Where Apple falls: Fending off a monopoly

If you’ve kept up with the company’s soap operatic feud with Adobe (ADBE), then you know how Apple (AAPL) feels about HTML5. Nowhere was the company’s position more crystal-clear than in Steve Jobs’ a 1,600-word missive dubbing HTML5 “the future” and Flash a buggy, battery-sucking relic of the past.

For Apple, preserving user experience is of utmost importance, but there are other less obvious ways HTML5 will help. Some may argue Apple doesn’t really gain anything from championing something it can’t possibly own, but think of it this way: if Apple can’t technically lay claim to it, at least no other company like Google, Microsoft, and Adobe can either. In doing so, Apple is arguably working towards wresting standards, APIs and development tools away from the competition.

“Apple is so confident in its ability to offer a superior user experience with the iOS platform and also with the Mac OSX platform, that having a web that is not run or monopolized by any company is the safer bet for them,” says Faruk Ates, a creative design and web consultant who worked at Apple for three years.

From a software perspective, Apple’s stance might seem counterintuitive, considering iTunes, which includes the (native) apps store, continues to thrive, with some 160 million registered users. With HTML5, it’s entirely possible that web-based apps could one day supercede native apps. Why push for a developer language that could one day affect the company’s bottom line?

Think of HTML5 as an on-ramp. Once developers adopt the easy-to-program language, they’re already halfway to developing natively for Apple. In general, some apps are already “hybrids,” built out of several web technologies with Objective C, that serves up Safari panels internally, which push the user out into the wild blue yonder. The more developers who think that way, the more Apple could potentially pull them the rest of the way in.

That of course, would be a company win for Apple. And an indication of how both Apple and Google, with very different operating ethos, can both get behind the same open standard– HTML5 — for different reasons, and both “win.”