Many clients have requested that their websites be built using HTML5 and CSS3.
More often than not, their reasoning for it is, because they hear it's the new kid in town and everyone wants to be it's friend.Although HTML5 and CSS3 are the powerful future of the web, we have to remember that not everyone has a browser that supports HTML5 fully, let alone partially.
Whenever it comes to using new technologies, it is important to know that it takes time for internet users to upgrade.As developers, we are at the mercy of the users, not the other way around. And this is how it should be.
After all, we are developing websites and web apps for users. It is their experience that matters, not ours.
So when it comes to HTML5 and CSS3, let's remember that every browser implements the standard differently (even if slightly) and thus a lot more work (time) is required to make sure the site functions as it should on all browsers and falls back properly when no HTML 5 support is available.
This reminds me of the days where we had Flash and Non-Flash versions of websites. Now we have HTML5 and Non-HTML5 versions. Another mess!