The unsung hero of the web has always been Javascript, without which the standards-based web would be completely static. Javascript enables functionality to be executed in the browser, and has been used to create all sorts of effects otherwise not possible with HTML alone.
In the early days, Javascript implementations weren’t entirely standard, requiring developers to have to write variants for different browsers; this isn’t really an issue any more.
For applications, developers will either use libraries or develop their own validation routines. This Javascript code adds significantly to the amount of code downloaded.
With HTML5, developers will need to write less Javascript, as the browser provides features to do things for itself rather than rely extra scripting.
Validation is the main area of improvement. HTML5 now provides a number of new validation features such as mandatory checking, type checking, range and field length validation. The validation is done within the browser, and developers can opt to decide how to process errors.
Obviously validation has to be repeated on the server for security, to ensure that data hasn’t been hacked in the browser or in transmission. This then means that validation has to be maintained in two places and kept in sync.
HTML5 also provides a number of new input field types such as tel, email, color, datetime. This empowers the browser, by applying it to display a date picker, or a colour chooser for example. More importantly for mobile applications it would allow the browser to show an appropriate keyboard layout e.g. a numeric layout for tel, and an alphabetic keyboard for email type.
There are also a number of new attributes which previously required Javascript such as autocomplete, placeholder and pattern which will prove very useful.
There will be some organisations that will not want the browser to affect their carefully designed user experience; for these people the answer is simple, just don’t use the new features.
For the rest, you will enjoy having to write less Javascript for HTML5 browsers, but of course you will still need to have backwards compatibility for non-HTML5 browsers which will rely on Javascript.