Microsoft has spent the past few days talking about the new browser formerly known as Project Spartan: what it will do, what it won't do, and what it won't do yet but will do soon.

We already knew that Microsoft Edge would remove much of the legacy technology that's found in Internet Explorer. Microsoft has given perhaps the fullest rundown of what's not in Edge this week. The two traditional ways of extending Internet Explorer, ActiveX and Browser Helper Objects, are both gone. This means no plugins, no toolbars, no Java, no Silverlight. It doesn't, however, mean no Flash; that's a built-in capability. PDF rendering is also built-in.

In their place are Chrome-like extensions built in HTML and JavaScript. However, these aren't coming immediately. Although Microsoft has demonstrated the popular Reddit Enhancement Suite running in Edge with (the company says) minimal changes from its Chrome version, the initial release of Edge won't support these extensions. There's no specific timeline on when they'll be added.

The new extensibility support will be quite broad. Internet Explorer currently has lots of extension points for developers; they can add, for example, custom download managers, custom protocol handlers, context menu entries, sidebars, and security filters. All of these and more will be handled by the new extensibility system when it's available.

The company has also said that it has a "long-term goal" of bringing extension support to its mobile browser, though initial support will be for PC only. More specifically, it will be for Windows 10 only. There are "no plans" to make the browser (or its core engine) open source, and doing so would apparently come at "massive cost."

Unlike Internet Explorer, Edge won't try to mimic older browsers in order to work around page bugs and glitches. This means that document modes and layout quirks are both gone. Edge will always be at the cutting edge, offering Microsoft's newest take on Web standards. This commitment to standards also means that various non-standard technologies are being removed: Edge won't support VML vector graphics, VBScript scripts, DirectX filters and transitions, or non-standard scripting techniques for responding to events or accessing CSS styling.

In Edge, Microsoft is also committing to not adding new proprietary stuff in the future. Significantly, this means that it's no longer going to use the vendor prefix system for providing early access to features that are still experimental or in the process of being standardized. Instead, developers will have to enable experimental features using configuration flags. Microsoft has also proposed development of a system in which browser developers could, in a limited way, enable trial usage of experimental features so that new capabilities can be tested "in the wild," but in such a way that doesn't allow experimental or non-standard features to become entrenched.

Joining extensions in the "planned for some time after the Windows 10 release" timeframe are support for the Object RTC specification, used to build realtime voice and video communications in the browser; Pointer Lock, used to constrain pointer movement (important for gaming); and a greater variety of Cortana scenarios.

As for a final feature that should make it in the first version, Microsoft announced back in February that it was investigating adding support for asm.js, the high performance JavaScript subset, to its Chakra JavaScript engine. The Edge build released in Windows 10 build 10074 last week includes experimental asm.js support (though it has to be enabled manually), and some benchmark scores published by Microsoft suggest that it can provide some huge performance boosts.

In a WebGL benchmark using the Unity 3D engine, Edge without asm.js support is about 50 percent faster than Internet Explorer 11. Turn on the asm.js feature, and performance doubles, making it three times faster than the old browser. Physics simulations and artificial intelligence showed the biggest gains.

All told, Edge is shaping up to be a very different browser from Internet Explorer 11. Microsoft says that in total, some 220,000 lines of code, and 300 old APIs, have been removed from Edge. 300,000 new lines of code have been added, with more than 4,200 fixes made to improve Edge's interoperability and compatibility with other browsers. Soon the only thing it'll have in common with its ancestral predecessor will be its blue "e" icon.