Attributes in elements closing tag?

Is it considered valid to do the following:

<li>stuff</li class="randomlengthclassname">
<li>stuff</li class="shortclassname">
<li>stuff</li class="reallyreallylongarseclassnam开发者_如何学编程e">

or do the attribute have to be in the opening tag?

No it isn't. You must use attributes in the opening tag.

Running <a>test</a href="tst.html"> in w3c validator results in this error:

name start character invalid: only S separators and TAGC allowed here

Where S separators and TAGC are:

    S is "whitespace" separator

    [5] s =

        SPACE | (32) space
        RE | (13) CR
        RS | (10) LF
        SEPCHAR (9) HT

            -- http://xml.coverpages.org/sgmlsyn/sgmlsyn.htm#C6.2.1

    TAGC    ">"

            -- http://www.w3.org/TR/sgml.l

This is not valid, and all attributes must be defined in the opening tag, indeed.

The attribute has to be in the opening tag. The code which you have presented probably wouldn't work.

Attributes should appear in the element's start tag. Quoting the W3C: On SGML and HTML Attributes:

... Attribute/value pairs appear before the final ">" of an element's start tag. Any number of (legal) attribute value pairs, separated by spaces, may appear in an element's start tag. They may appear in any order.

Closing tags may not contain attributes. But in HTML4 you may omit the closing LI:

<!ELEMENT LI - O (%flow;)*             -- list item -->
<!ATTLIST LI
  %attrs;                              -- %coreattrs, %i18n, %events --
  >

Start tag: required, End tag: optional

In XHTML, you may not.