Browsium, the Redmond startup whose technology lets companies run legacy web apps in the latest versions of Microsoft’s Internet Explorer and Windows, has signed its biggest customer to date, the London-based HM Revenue & Customs governmental agency.

Computer Weekly reports that the agreement “will act as a proof of concept for other government departments facing similar upgrade problems” — signaling that the deal could lead to further business for Browsium.

The news site pegs the value of the agreement at £1.28 million, or more than $2 million.

The announcement follows Browsium’s release of a second-generation product called Browsium Ion that uses specialized settings and other techniques in Internet Explorer 8 and 9 to run apps that were originally written for Internet Explorer 6 and don’t normally run properly in the newer IE versions.

Browsium Ion can switch the settings automatically on the fly, under the hood, so that users can also run new web pages and apps. The approach addresses one of the major roadblocks to companies upgrading to Windows 7, which doesn’t support Internet Explorer 6.

Even so, Microsoft has made it clear would prefer to see businesses rewrite or replace their legacy apps to work on newer versions of the browser and the operating system.

Browsium’s executives include president Gary Schare, the former head of the Internet Explorer product management team. The company was founded by Matt Heller, a former Overture Services executive who also worked previously with Microsoft on Internet Explorer. The company’s CTO is Matt Crowley, a former program manager on the Internet Explorer and ASP.NET teams.

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