[TO] HTML5

[Ken Corey]

>>> * When will the fact that my website is NOT written in HTML5 will
>>> browsers
>>> be unable to cope with it? *

What you're asking:
Whicih is better: HTML4 or HTML5?

The question needs to be refined.  It's like asking:
Which is better: Mac or PC?  Emacs or Vi?

There's no point in these questions, and nobody here is going to be able 
to give you useful input.

The better question is:
What version of HTML would I get the best return from?  That you can 
attempt to calculate in terms of money.

Translating to HTML5 has a cost in development time.  What that cost is 
depends on the current state of your web site, and what state to change 
to.  You haven't told us either of these at all, so there's no way for 
us to make suggestions on this point.

Not translating to HTML5 perhaps has a cost in lost opportunities, 
(though I don't know what those might be for you), or lost customers 
whose browsers don't handle HTML5 (so you'd have a cost to 
regain/convert/find new customers)

You need to either take all the measurements of browser types and OS 
used on your site to try to answer this one.  Without knowing what 
percentage of your user base uses old browsers, how could anyone on this 
list give you an answer?

Best of luck.

-Ken