Exercise 1: A div Whose Only Child Is a Comment — Possible Solution
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THE TEST
------------------------------
    only_comment_parent = Element('div', {})
    only_comment_parent.children = [CommentNode(' just a note, nothing else ')]
    assign_parents(only_comment_parent)

    styled = build_style_tree(only_comment_parent, Stylesheet([]))
    len(styled.children)

RESULT
------------------------------
    0 -- styled.children is an empty list. No crash, no placeholder node.

WHY THIS IS THE CORRECT RESULT
------------------------------
build_style_tree(only_comment_parent, ...) takes the 'element' branch:
it computes a style for the div itself, then loops over its DOM
children (just the one CommentNode) and calls build_style_tree on each
one recursively:

    for child in dom_node.children:
        styled_child = build_style_tree(child, stylesheet, style)
        if styled_child is not None:
            children.append(styled_child)

The recursive call on the CommentNode hits the very first check in the
function:

    if dom_node.kind == 'comment':
        return None

So styled_child comes back as None, the `if styled_child is not None`
guard correctly skips appending anything, and the loop finishes having
added nothing at all to children. The div's own StyledNode is still
built and returned -- it's a real, present node in the tree -- it
simply has an empty children list, because none of its DOM children
survived being converted.

WHY THIS WORKS AS AN ANSWER
------------------------------
This confirms comment-exclusion and "an element can have a StyledNode
with zero children" are two independent, compatible behaviors that
combine cleanly: the PARENT is never punished for the fact that all of
its own children happened to be non-renderable content. Nothing about
build_style_tree assumes an element with DOM children must also end up
with StyledNode children -- the two counts (raw DOM children vs.
kept StyledNode children) are allowed to diverge freely, and an empty
children list is a perfectly ordinary, valid outcome, not an error
condition the function needs to special-case or guard against.