Some clients are a bit afraid of using HTML5:

  • "Why should we follow the standard that hasn’t even been finalized yet?"
  • "What if one or some of the big players (like Oracle or Microsoft) decides to give it up and bring up their own standard?"
  • "What it HTML5 evolves that heavily that we will have to constantly modify our applications to remain complaint with the browsers?"
Yea, well. In theory, the standard is supposed to be finalized in 2014, but some say that it won’t be in 100% adopted by all commonly used browsers (in all versions) until 2022! Does it mean we have to wait that long? Of course not. Fortunately, HTML5 is not a monolith but rather a list of independent features. Their independence is crucial here as HTML5 may be implemented partially and the clue is that we use this implemented parts :)
To check the current situation (and how it changed in time) look at:
If you need more detailed info, here’s your spot:
My recommendation? There’s just too much goodness in HTML5 to give it up that easily, especially if you see how well adopted (in general) HTML5 is. Keeping in mind that there are already present libraries like Modernizr (http://modernizr.com/) and some very nice fallbacks for missed features - go for it!