UI Browser is not the tool for browser content (it can interrogate only standard-issue macOS UI controls)
On Chrome, the usual place to start would be to:
-
ctrl-click
the element of interest (to get a contextual menu) - Choose
Inspect
You should then get a sense of which html elements are in play, and which CSS styles are applied to each.
Another approach would be to copy some formatted text from the target segment editor, and then see what you've got in the clipboard
(using a clipboard viewer macro like this: Clipboard Viewer - Macro Library - Keyboard Maestro Discourse)
Or in Apple's Additional tools for XCode
https://developer.apple.com/download/all/?q=clipboard%20viewer