I tried to explore the synchronous constructor option a bit more. (See https://wiki.whatwg.org/wiki/CustomElements for details.)
>From an HTML parser perspective the main problem would be that elements with a dash would have to go into the same path that is basically a special case for </script> today. So the performance of the parser would suffer. It's unclear how much of a hit that would be. Only </script> causes synchronous invocation of script. Otherwise script is invoked based on a timer or lack of incoming data from the network to process. This would change that as elements with a dash could cause synchronous invocation of script as well. This would give custom elements the same freedom as builtin elements. We could even add a hook for popping of the end tag so you can emulate the behavior of these elements: script, style, object, video, and audio. I don't think there's a way to do that with the alternative strategies. (I checked and Servo has no plans to parallelize tree building.) -- https://annevankesteren.nl/
