Microsoft recently revealed that it’s officially ending Internet Explorer (IE) with the launch of Windows 10. The venerable IE will be replaced by the streamlined new Project Spartan browser. Many are ecstatic about the demise of IE, and some are skeptical that Project Spartan is anything more than IE with a new coat of paint. However, the one thing Project Spartan really needs to capture market share has little to do with the browser itself: it needs to be cross-platform.

Microsoft has already conceded the cross-platform war. Its recent strategy of offering OneNote, followed by the full Microsoft Office productivity suite, on iOS and Android mobile devices has been effective. Last week, it added its Office Lens app for iOS and Android to extend the ability to scan documents and whiteboards from a mobile device on rival platforms. It needs to adopt that same all-inclusive mindset with Project Spartan.

Project Spartan cannot be a Windows-only browser like IE. Microsoft needs to make Project Spartan for Mac OS X, Project Spartan for iOS, Project Spartan for Android–maybe even Project Spartan for Linux. Much of the value of a browser today lies in the ability to open a tab in the browser on your PC and pick up where you left off from your smartphone while you’re on the go. People expect tabs, bookmarks, and even logins and passwords to be seamlessly synced. People want a browser experience that remains consistent no matter which device or platform they’re on.

IE once enjoyed a virtual monopoly of the browser market. Firefox and Chrome–and to a lesser extent Apple’s Safari–have eaten away at that dominance. IE still has more market share than all of those rivals combined, but its 90+% market share dwindled to just over 50%. Even that figure is debatable, because it depends on how you measure the browser market. There are metrics that suggest Chrome is more popular than IE.

Part of that is driven simply by the browser itself. Firefox and Chrome both have vibrant, dynamic ecosystems of extensions and plug-ins that enable the browser experience to be customized and more powerful than the off-the-shelf browser itself. IE also has add-ons available, but they’ve never had the loyal following of Firefox and Chrome.

Another factor in the fall of IE has been public relations. IE has had its share of security and performance issues. Typically, a vendor will address such issues, and it becomes a faded memory. With IE, however, there has been a sustained campaign of negative marketing and media attention. An entire generation hates IE and refuses to use it, and they don’t even really know why. They just know that everyone says you should avoid IE.

Project Spartan can help Microsoft start fresh. It can reinvent the Microsoft browser as a more flexible and efficient browser experience. It can shed the negative stigma of the IE brand. It will all be for nothing, though, if Microsoft doesn’t also make Project Spartan available as a cross-platform browser that enables users to keep tabs and bookmarks synced and remain productive as they switch from Windows PC, to iPhone, to Android tablet.

It’s a multi-device, cross-platform world. Project Spartan needs to recognize that, or it’s DOA.