"The reply button, for example, focuses and expands the textbox below. You can’t do that without some scripting" -- Can be done with just CSS :target

"Similarly, the button does an action behind the scenes, which is iffy since you could replicate it with a full page load" -- Should just be a normal link with a full page reload if JS disabled. Perhaps this could also be a new use of the <a ping> attribute (without a href).