But consider the following problem: Find and display all comments by me that are children of this post, and only those comments, using only browser UI elements, i.e. not the LW-specific page widgets. You cannot -- and I'd be pretty surprised if you could make a browser extension that could do it without resorting to the API, skipping the previous elements in the chain above. For that matter, if you can do it with the existing page widgets, I'd love to know how.

This is actually trivial (but it breaks readily if LW changes it's stylesheet):

from lxml import html
root = html.open("<http://lesswrong.com/lw/njn/the_web_browser_is_not_your_client_but_you_dont/>").getroot()
for div in root.cssselect(".comment-meta"):
if div.cssselect("span.author")[0].text_content() == "Error":
print div.text_content()