Most of the time, the developers would want to use their existing skills and develop more apps targeting different and multiple platforms. For example, the Web developers might want to develop Windows phone apps using HTML, CSS, Java Script and other Web technologies that they know.
Can the Windows Phone 8 app be developed using Web technologies like HTML5, CSS3 and JavaScript?
Yes, the Web developers can utilize their web development skills (HTML5, CSS3 and JavaScript) and develop Windows Phone apps. The Windows Phone 8 SDK provides an HTML5 App template which lets the developers to develop the Mobile App with these technologies.
There is an interesting video in Channel 9 MSDN which explains Developing in HTML5 and JavaScript for Windows Phone.