This is the first announced partnership in what Microsoft describes as a push to open up its platform and create a "more impressive Web experience in Windows 10." From Microsoft:
Adobe is a major contributor to open source browser engines such as WebKit, Blink, and Gecko. In the past, it was challenging for them (or anyone external to Microsoft) to make contributions to the Internet Explorer code base. As a result, as Adobe improved the Web platform in other browsers, but couldn't bring the same improvements to Microsoft's platform. This changed a few months ago when Microsoft made it possible for the Adobe Web Platform Team to contribute to Project Spartan. The team contributes in the areas of layout, typography, graphic design and motion, with significant commits to the Web platform. Adobe engineers Rik Cabanier, Max Vujovic, Sylvain Galineau, and Ethan Malasky have provided contributions in partnership with engineers on the IE team.
Microsoft goes on to talk about contributions from Adobe that are already available in the Windows 10 March Technical Preview that include support for CSS features such as gradient midpoints and new blend modes for images.
While today's blog post from Microsoft only mentions Adobe, it sounds like we'll hear more about other partners in the coming months. In any event, the full post is a good read, and you can check it out at the source link below.