Why does the align attribute doesn't work with the HIGHLIGHTS like h1,h2 and so on???

Question

Like when i write
<p align="center">First text</p> --This works right.
<p align="center"><b>First text</b></p>--This too works right.
BUT
<p align="center"><h1>First text</h1></p>--This doesn't work.
CAN ANYONE EXPLAIN IT TO ME??