I edited
\assets\minecraft\textures\gui\icons.png &
\overlay_18\assets\minecraft\textures\gui\sprites\hud\experience_bar_progress.png
to 10% transparency, but
in 1.20.1 xp bar can not be semi transparent
in 1.20.2 all elements can't be semi transparent
I want to be able to use semi transparent HUD resourcepacks

Edited: other elements can be transparent, just version mismatch problem. But XP bar still unable to be semi transparent.