Using the elytra, even though you would usually have to "place" fireworks", with the elytra you can just use fireworks in midair. You can do this even above building height limit. While I'm not certain this is a bug, it allows you to go extremely high ridiculously fast.
What I expected to happen was...:
I expected that once I went above the building height limit, trying to use a firework would just display the building limit reached message. One reason I thought this is that, if you try to change a block (such as a repeater) that changes state by right clicking, which itself is below the limit but you are above, displays the message. (This in itself might be a separate bug). Another reason I thought it wouldn't work is that, when placing a firework without using an elytra, if you do it above the limit it doesn't work.

What actually happened was...:
The firework was launched as normal.

Steps to Reproduce:
1. Equip an elytra
2. Give yourself some fireworks.
3. Using those fireworks, go above the building height limit.
4. Try using one above the limit, and it will still work if the elytra is active.