Dear Ppl:
I am attempting to be a calibre developer

I really wonder how calibre does to get corresponding css by clicking one point in html editor.
my questions are as follows:
1. how you match corresponding css given certain element node in the element tree? some selector match engine? like tinycss2 library?
2. (most important one

how does calibre do to get a element node by just simply clicking one line in html editor, how does it iterate the tree to get its element node by just clicking?
Any hints or tips will be soo appreciated!
Best regards.