Web developers, until now, would now be able to have their creations run after a user locks their screen. The application would have to get explicit permission by the user to run after the screen is locked. This has now changed. Microsoft has changed their policy, which will now permit applications to run with locked screens so long as there is enough battery life on the device. This change in policy and approach is a matter of convenience for users. Audio applications are a good example of the type of application that a user would wish to run with a locked screen.
This issue has been a long running complaint with mobile, application developers--the fact that Microsoft's own applications can run in the background, but third party applications could not with much ease.