I've tried checking other answers, but I'm still confused--especially after seeing W3schools HTML 5 reference.
I thought HTML 4.01 was supposed to "allow" single-tags to just be <img> and <br>. Then XHTML came along with <img /> and <br /> (where someone said that the space is there for older browsers).
Now I'm wondering how I'm supposed to format my code when practicing HTML 5.
<!DOCTYPE HTML>
Is it <br>, <br/> or <br />?
xml doesn't allow leaving tags open, so it makes <br> a bit worse than the other two. The other two are roughly equivalent with the second preferred for compatibility with older browsers. Actually, space before / is preferred for compatibility sake, but I think it only makes sense for tags that have attributes. So I'd say either <br/> or <br />, whichever pleases your aesthetics.
To sum it up: all three are valid with the first one being a bit less "portable".
Edit: Now that we're all crazy about specs, I think it worth pointing out that according to dev.w3.org:
Start tags consist of the following parts, in exactly the following order:
Simply <br> is sufficient.
The other forms are there for compatibility with XHTML; to make it possible to write the same code as XHTML, and have it also work as HTML. Some systems that generate HTML may be based on XML generators, and thus not have the ability to output just a bare <br> tag; if you're using such a system, it's fine to use <br/>, it's just not necessary if you don't need to do it.
Very few people actually use XHTML, however. You need to serve your content as application/xhtml+xml for it to be interpreted as XHTML, and that will not work in IE (it will also mean that any small error you make will prevent your page from being displayed, in browsers that do support XHTML). So, most of what looks like XHTML on the web is actually being served, and interpreted, as HTML. See Serving XHTML as text/html Considered Harmful for some more information.
I think this quote from the HTML 5 Reference Draft provides the answer:
Some elements, however, are forbidden from containing any content at all. These are known as void elements. In HTML, the above syntax cannot be used for void elements. For such elements, the end tag must be omitted because the element is automatically closed by the parser. Such elements include, among others, br, hr, link and meta
HTML Example:
<link type="text/css" rel="stylesheet" href="style.css">In XHTML, the XML syntactic requirements dictate that this must be made explicit using either an explicit end tag, as above, or the empty element syntax. This is achieved by inserting a slash at the end of the start tag immediately before the right angle bracket.
Example:
<link type="text/css" href="style.css"/>Authors may optionally choose to use this same syntax for void elements in the HTML syntax as well. Some authors also choose to include whitespace before the slash, however this is not necessary. (Using whitespace in that fashion is a convention inherited from the compatibility guidelines in XHTML 1.0, Appendix C.)
If your interested in comparability I'd stick with <br /> ifnot <br> is fine
XML requires all tags to have a corresponding closing tag. So there is a special short-hand syntax for tags without inner contents.
HTML5 is not XML, so it should not pose such a requirement. Neither is HTML 4.01.
For instance, in HTML5 specs, all examples with br tag use <br> syntax, not <br/>.
UPD Actually, <br/> is permitted in HTML5. 9.1.2.1, 7.