Safely use HTML 5 and CSS 3 Today with Modernizr
The principal problem with using HTML 5 and CSS 3 isn’t the adoption rate or the differences between browsers, it is knowing what those differences are in the first place. Once that is known developers can work around the limitations using graceful degradation techniques. To help figure that out many turn to the open source project Modernizr.
Rather than look at user-agent strings, Modernizr uses a series of tests to determine the capabilities of a browser. In a few milliseconds it can perform over 40 tests and record their results as properties on an object called Modernizr. Using this information developers can detect if a feature they are using isn’t supported and react accordingly.
In order to improve performance, Modernizr can be customized to only perform the tests that are needed by the site. This is done via the Modernizr download page, which also services as a list of detectable features. There is a separate list of undetectable features and possible workarounds on their github site.
Beware of polyfills and JS shims