With its increased mobile support, HTML5 development gives developers access to a variety of advanced web application features that are common across many different mobile platforms and operating systems; including Windows Phone.
Windows Phone and IE Mobile support all the stable HTML5 specs from W3C Standards which allows developers to build modern mobile web sites and HTML5-based applications using third-party frameworks and delivering an advanced and integrated experience to the final users.
The first place you should visit is MSDN that offers plenty of information and resources to help you develop sites that will render on IE 9, including IE9 Mobile which is the mobile browser you can find in Windows Phone 7.5.
To find some other interesting resources, you can visit Rob Tiffany's blog who offers a simple boilerplate for HTML5 on Windows Phone, and you can find detailed samples and demos of HTML5 on IE9 Mobile on the IE testdrive site for Mobile.
Developing HTML5 based applications for Windows Phone
In order to better integrate the hybrid applications developed with these frameworks into the Windows Phone experience a jQuery Mobile Theme for Windows Phone (Metro Style) is also available which adapts the appearance of HTML5 elements and controls to provide a consistent user experience.
To learn more about Apache Cordova for Windows Phone, you can read the following articles:
In order to dive in the the jQuery Mobile framework development for Windows Phone, here are some guides for you:
The community is sharing some tools that can be of a good help when developing HTML5-based applications for Windows Phone, like this mobile IE Debugger tool.