Note that in Servo a lot of reflows can be done off the main thread, not synchronously. Don't take this list as something that will be true for all time.
Even if the APIs in question require synchronous reflows now because they return some data, it may be possible to introduce new APIs that receive the data in a callback. (The usefulness of these APIs depends on the engine having support for off main thread layout, of course.)
Yes! This has me quite excited about Servo. Chrome is interested in async APIs for these as well.
Random aside: Dart's DOM originally offered these properties only as async but developers were unhappy with the ergonomics so Dart ended up delivering the sync APIs. :/
Even if the APIs in question require synchronous reflows now because they return some data, it may be possible to introduce new APIs that receive the data in a callback. (The usefulness of these APIs depends on the engine having support for off main thread layout, of course.)