Microsoft's new Project Spartan browser isn't available just yet, but but there's a way to test sites for it inside the Windows 10 technical preview's version of Internet Explorer.
In the latest Windows 10 Technical Preview build, users can toggle a hidden flag in Internet Explorer to enable Spartan's new Edge rendering engine. Though it's still experimental, it should allow you to test websites and other online tools to see how they're rendered by what will be the default brower for Windows 10 users.
There will still be a version of Internet Explorer in Windows 10, but according to Microsoft's Jacob Rossi, that's just for users who have to access corporate intranets and other web services that require outdated components that won't be in Project Spartan (yeah, you, VBScript).
How to enable the Spartan Edge rendering engine in IE (Internet Explorer)
To enable the new engine, just type "about:flags" in IE11's address bar, select "Enabled" under "Enable Experimental Web Platform Features," and restart the browser. Enabling the "Custom User Agent" option will trick sites that nerf IE specifically into using the Spartan Edge rendering engine.
Benchmarks by AnandTech show better scores for Edge in all major benchmarks compared to the old IE engine. Edge also outperforms Google Chrome and Mozilla Firefox in a couple of tests (Sunspider and Octane), though it still lags behind in others (Kraken, WebXPRT, Oort Online, HTML5Test).
In addition to the performance improvements, Project Spartan will bring new features such as webpage annotation, Cortana integration, a built-in offline reading mode, and the ability to save page clippings to OneNote. It also has a slick new look, with a tabs-on-top layout similar to other browsers. The new browser should show up in a future Windows 10 preview build, but the timing is unclear.