Many developers fail to constantly test their work in multiple web browsers during HTML5 development, either because they are too lazy to spend the time to do it, or they just all together forget. Easily one of the biggest issues you will encounter when testing and publishing your final application for public usage will be browser support. If you didn't manage to test your project in various browsers on different platforms, you may be surprised when a user complains that your application doesn't work for them. As HTML5 still is the new kid on the block, many browsers are still running to catch up with the feature support needed to display your HTML5 content properly.
Keeping up-to-date with what current web browsers support in the realm of HTML5 feature support can be an extremely important asset to gain as a developer. Keeping yourself and your project out of impending doom, because you were able to spot a lack of support on a specific device or platform...