Microsoft confirmed last week what had been rumored months before: Windows 10 would include a new browser codenamed Project Spartan designed for the modern Web.

At the event, Spartan's new user interface was shown off. Beyond regular browser capabilities, Spartan will include Cortana integration, a note-taking capability, and a reader mode.

Since that announcement, a few more details have emerged, painting a clearer picture of what Microsoft is doing with its new browser.

The heart of the browser is its rendering engine. Microsoft first described its new engine last year. The company isn't abandoning its Trident engine in favor of WebKit (used in Safari) or Blink (used in Chrome and Opera). Instead, it has forked the engine to create two versions. There's the legacy Internet Explorer 11 engine that can mimic old browsers as far back as Internet Explorer 5.5, and there's a new version, rebranded Edge, that strips out all this backwards compatibility.

Internet Explorer Program Manager Jacob Rossi described the changes in a little more detail earlier this week. Microsoft took a hatchet to the Trident codebase and cut out large chunks of it. Vestigial features included only because old pages depended on them were removed; the Internet Explorer 6-esque interpretation of standards known as "quirks mode" has been purged; non-standard features such as VBScript support have been banished.

Microsoft also removed certain transitional features that it had developed. In Trident, Web developers could force the engine to behave a particular way through the use of certain HTTP headers, enabling them to both force older or newer behavior as necessary. This is gone from Edge: Edge always uses the newest implementation and interpretation of the standards.

It seems as if the work has been considerable. Rossi said that Edge has diverged further from Trident than Google's Blink has from the WebKit that it is based on.

The need for Trident's legacy compatibility hasn't gone away. It is, however, greatly reduced, such that it's no longer necessary or desirable to use it as the default. The scourge of "Designed for Internet Explorer 6" is now a relic of history (though some developers are unfortunately trying to resurrect it), and so Microsoft's browser no longer needs to live in the past.

To do this, however, Edge needs to make sure that Web content no longer treats it as if it were living in the past. To that end, it has a new and rather surprising user agent identification string:

"Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.71 Safari/537.36 Edge/12.0"

The key part is the "Chrome" bit: Edge is most likely to be presumed to be Chrome (or at least, closely related to Chrome), rather than Internet Explorer. The Edge/12.0 at the end is the only bit that discloses the true nature of the browser.

The new engine should also reduce the impact of compatibility features in the future by substantially ditching the "prefix" mechanism. For a time, experimental JavaScript and CSS features were exposed by browsers to developers by using browser-specific prefixes; "-ms" for Internet Explorer, "-webkit" for Safari, and so on and so forth. This has created a significant compatibility headache for browser developers, because sites using those prefixes have become abundant on the Web. This in turn makes it difficult for browser developers to either change those prefixed features (to accommodate updated specifications, for example) or drop those prefixes entirely once features leave the experimental stage.

In Edge, experimental features won't use prefixes. Instead, they'll have to be manually enabled by end-users on a flags screen. This means that Web developers can try out the experimental capabilities and provide feedback on them, but they can no longer leak onto the real Web.

As well as stripping out the legacy code, Microsoft has added a bunch of new capabilities, including support for HTTP Strict Transport Security, that tells the browser that some domains should always be visited using SSL, HTTP Live Streaming and Dynamic Adaptive Streaming over HTTP, for better plugin-free video playback, the Gamepad API, and various features that are set to be standardized in ECMAScript 6.

Indications are that the new engine is fast, too. Trident and its Chakra JavaScript engine have long been good performers at the SunSpider benchmark, but early benchmarking by AnandTech suggests that Edge and Chakra show a marked increase in the Octane and Kraken benchmarks, too. The improvement in Google's Octane test is particularly big: it runs more than 80 percent faster in Edge than in Trident, bringing Spartan's performance in line with Chrome's, and faster than Firefox's.

This advantage should in principle extend not only to the Web but also to Windows apps built using HTML5.

While most of the Web can use Edge mode, legacy content isn't gone entirely. It's particularly abundant in corporate intranets and other controlled environments. Microsoft's approach to this is two-fold. First, the Spartan browser will be able to use both Edge and Trident. Edge will be the default for the Web; Trident will be the default for the intranet.

That will address many, but not all, concerns. Even when using Trident, Spartan will omit some legacy capabilities, such as support for Internet Explorer's third-party toolbars and ActiveX plugins. For these, Internet Explorer will continue to be required, and accordingly, Windows 10 will ship with Internet Explorer 11 for those situations that absolutely require it.

While it's ditching those old extensibility mechanisms, Microsoft confirmed on Twitter that there will be an extension capability of some kind. Microsoft is yet to provide any specific information, though the rumor mill continues to suggest that the system will be compatible with Chrome's extension system.

In a message that's not altogether surprising but will no doubt cause disappointment, Microsoft has also said that Spartan (and, one assumes, Edge) is a Windows 10-only affair, with no plans for a Windows 7 release. This is to be expected: Windows 7 left mainstream support earlier this month, meaning that it's only receiving security updates. Significant platform updates—such as a major overhaul/replacement of Internet Explorer—are off the cards.

With Windows 10 available as a free upgrade to both Windows 7 and Windows 8 users, Microsoft will likely argue that this is enough and that there's no need to make the new browser available on the old platform.

Of course, if the company released Edge as open source, which we argue it should do, then it could rely on an industrious third party to provide Windows 7 support. Such a move would keep those users happy while requiring relatively little effort (or support) on Microsoft's part. Refusal to offer new browsers on old platforms has long been a complaint leveled at Microsoft; open sourcing the engine would provide some sort of long-term solution.

One thing we still don't know is what Spartan will actually be called when it ships. Windows Central reported that Microsoft had tested a number of names, including "Edge," possibly pairing those names with "Internet Explorer" or "IE." So perhaps the Internet Explorer name will continue after all.