We resolved to make this undefined for Level 3, since we weren't sure what the correct behavior should be, but we might want to consider defining it more precisely in Level 4 if there's sufficient information on the right behavior.

Thread:

• https://lists.w3.org/Archives/Public/www-style/2016Mar/0073.html
• https://lists.w3.org/Archives/Public/www-style/2016Sep/0097.html
• https://lists.w3.org/Archives/Public/www-style/2016Mar/0074.html