Well-Formedness - Refactoring HTML: Improving the Design of Existing Web Applications - page 78

<embed >
<param>
<area>
<frame>
<col>
<input>
In addition, a few other elements from various proprietary browser extensions may also appear:
<basefont>
<bgsound>
<keygen>
<sound>
<spacer>
<wbr>
Although XML and XHTML allow these tags to be written either with a start-tag/end-tag pair such as <br></br>
or with an empty-element tag such as <br /> , the latter is much friendlier to older browsers and to human
authors. There's little reason not to prefer the empty-element tag.
However, even an empty-element tag such as <br/> can confuse some older browsers that actually read this as
an unknown element with the name br/ instead of the known element br . Maximum compatibility is achieved if
you add an attribute and a space before the final slash. The class attribute is a good choice. For example:
<br class="empty" />
<hr class="empty" />
I picked empty as the class to be clear why I inserted it. However, the value of the class attribute really doesn't
matter. If you have reason to assign a different class to some or all of these elements, feel free.