Can I Use HTML5 Even if Users Have Outdated Browsers?

offcourse you can do this if you adopt best approach & use the language intellegently there is various approach like you can write browser specific html/css code with targeting each & specific browser there some helpful pre-written code like biolerplate, frameworks that help you to target specific browser that dosen't support html5 & CSS3. I am not a big fan of hacks but as last resort you can use it rare time.

At last you have to be very precise make the best decision thoroulgy examin your code on various browser & browser testing tools (install the VM for cross browser testing particularly for IE).

i can't use it cous there is no html5 in old browsers

long story short: No

i think no but you shuld update the browser to use html5