Mac and Linux users anxious to run Google’s Chrome browser now have a hard deadline for when that will be possible: the middle of 2009.

“That's what we've been hoping for,” said Chrome product manager Brian Rakowski, in an interview, describing both efforts as proceeding “in parallel” and “at the same level of progress.”

A crude “TestShell” version of the browser is already available on both platforms, but its functionality is minimal at best: the Chromium developer site describes it as a “single-process test harness” for browser development, allowing developers to better debug and test the browser’s page renderer before it is incorporated into Chrome’s famous but difficult-to-debug sandbox architecture.

According to the developer page, Chrome’s Mac and Linux renderer already passes 90% of the “all-important” WebKit layout tests, with outstanding problems in many cases related to text fields.

The weekend also brought news for Chrome’s Windows users: Google released an alpha build of Chrome 2.0 – less than a month after taking Chrome 1.0 out of beta and a few weeks over three months since its surprise release – and it includes a handful of new features, like autocomplete and user profiles, that bring it up to speed with Internet Explorer and Firefox.

More importantly, the Chrome 2.0 alpha supports “user scripts” much like the Greasemonkey plugin for Firefox: users can run custom Javascript code on a per-page basis, allowing customization of nearly any website on the Internet.

“We have user script support. That's a baby step,” said Rakowski. The Chrome development team will “expose more capabilities, then expose containers where can you have your own toolbar-like thing. You'll see it evolve over time.”

User scripts will lay the groundwork for full-blown Firefox-style extensions in the future, as right now the architecture for such a feature is simply not present. An informal CNET user survey, conducted last November, ranks Extensions as the third most requested feature for Chrome – trailing only the desire for Mac/Linux support and “faster performance”.

“Developer preview” versions of Chrome – one of the three update channels that users can subscribe to, with the others builds marked “beta” or the newly-minted “stable” status – will most likely be posted every couple of weeks, said Rakowski, while beta versions of the browser should appear monthly.