HTML5 Compliant - Trailing Space in Class Attribute

Question 1

As we can see in the link below, there is no restriction on what the developer can use in the class attribute.

http://www.w3.org/html/wg/drafts/html/master/dom.html#classes

In fact, after saying what the classes are and that they're used spliting in the spaces, the author(s) says:

There are no additional restrictions on the tokens authors can use in the class attribute, but authors are encouraged to use values that describe the nature of the content, rather than values that describe the desired presentation of the content.

Our colleagues here have tested and it successfully passes W3C validation, so I can't guess why your friend thought it was invalid.

Question 2

Yes, it is compliant.

From http://www.w3.org/html/wg/drafts/html/master/dom.html#classes:

The attribute, if specified, must have a value that is a set of space-separated tokens representing the various classes that the element belongs to.

From http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#set-of-space-separated-tokens:

A string containing a set of space-separated tokens may have leading or trailing space characters.

Question 3

According to http://validator.w3.org/ under a <!DOCTYPE html> the following validates successfully.

<div class=" name1 name2 "></div>

Leaving a trailing and leading space may be acceptable, but it's not pretty and some people do not consider it to be best a practice.

Question 4

Anything that knows how to handle class attributes should be fine with this. It is possible to give a tag multiple classes by separating them with spaces so anything which reads them had better know how to deal with spaces.