Odsud :

http://stackoverflow.com/questions/1946426/html-5- is-it-br-br-or-br

vyjímám:

In HTML (up to HTML 4): use <br>

In HTML 5: <br> is preferred, but <br> and <br /> is also acceptable

In XHTML: <br /> is preferred. Can also use <br/> or <br></br>

Notes:

<br></br> is not valid in HTML 5, it will be thought of as two line breaks.
XHTML is case sensitive, HTML is not case sensitive.
For backward compatibility, some old browsers would parse XHTML as HTML and fail on <br/> but not <br />