xhtml_conform-16: White space in attribute values is processed according to [XML].
ElementAttributeNMTOKENSCDATA

  1. All line breaks must have been normalized on input to #xA as described in 2.11 End-of-Line Handling, so the rest of this algorithm operates on text normalized in this way.

  2. Begin with a normalized value consisting of the empty string.

  3. For each character, entity reference, or character reference in the unnormalized attribute value, beginning with the first and continuing to the last, do the following:

    • For a character reference, append the referenced character to the normalized value.

    • For an entity reference, recursively apply step 3 of this algorithm to the replacement text of the entity.

    • For a white space character (#x20, #xD, #xA, #x9), append a space character (#x20) to the normalized value.

    • For another character, append the character to the normalized value.

If the attribute type is not CDATA, then the XML processor must further process the normalized attribute value by discarding any leading and trailing space (#x20) characters, and by replacing sequences of space (#x20) characters by a single space (#x20) character.

class X
style X
htmlxmlns X
inputvalue X
selectsize X
optionvalue X
textareacols X
textarearows X
td,thcolspan X
td,throwspan X
Tests for assertions xhtml_conform-16

class attribute: This must be italic

style attribute: this must be italic

input value attribute:

select size attribute   used for "2", both options should show:

textarea rows attribute: use Numeric character references for digits, the rows must be 20 and the cols 35: