getting HTML source or rich text from the X clipboard

In X11 you have to communicate with the selection owner, ask about supported formats, and then request data in the specific format. I think the easiest way to do this is using existing windowing toolkits. E,g. with Python and GTK:

#!/usr/bin/python

import glib, gtk

def test_clipboard():
    clipboard = gtk.Clipboard()
    targets = clipboard.wait_for_targets()
    print "Targets available:", ", ".join(map(str, targets))
    for target in targets:
        print "Trying '%s'..." % str(target)
        contents = clipboard.wait_for_contents(target)
        if contents:
            print contents.data

def main():
    mainloop = glib.MainLoop()
    def cb():
        test_clipboard()
        mainloop.quit()
    glib.idle_add(cb)
    mainloop.run()

if __name__ == "__main__":
    main()

Output will look like this:

$ ./clipboard.py 
Targets available: TIMESTAMP, TARGETS, MULTIPLE, text/html, text/_moz_htmlcontext, text/_moz_htmlinfo, UTF8_STRING, COMPOUND_TEXT, TEXT, STRING, text/x-moz-url-priv
...
Trying 'text/html'...
I asked <a href="http://superuser.com/questions/144185/getting-html-source-or-rich-text-from-the-x-clipboard">the same question on superuser.com</a>, because I was hoping there was a utility to do this, but I didn't get any informative responses.
Trying 'text/_moz_htmlcontext'...
<html><body class="question-page"><div class="container"><div id="content"><div id="mainbar"><div id="question"><table><tbody><tr><td class="postcell"><div><div class="post-text"><p></p></div></div></td></tr></tbody></table></div></div></div></div></body></html>
...
Trying 'STRING'...
I asked the same question on superuser.com, because I was hoping there was a utility to do this, but I didn't get any informative responses.
Trying 'text/x-moz-url-priv'...
http://stackoverflow.com/questions/3261379/getting-html-source-or-rich-text-from-the-x-clipboard

To complement @rkhayrov's answer, there exists a command for that already: xclip. Or more exactly, there's a patch to xclip which was added to xclip later on in 2010, but hasn't been released yet that does that. So, assuming your OS like Debian ships with the subversion head of xclip (2019 edit: version 0.13 with those changes was eventually released in 2016 (and pulled into Debian in January 2019)):

To list the targets for the CLIPBOARD selection:

$ xclip -selection clipboard -o -t TARGETS
TIMESTAMP
TARGETS
MULTIPLE
SAVE_TARGETS
text/html
text/_moz_htmlcontext
text/_moz_htmlinfo
UTF8_STRING
COMPOUND_TEXT
TEXT
STRING
text/x-moz-url-priv

To select a particular target:

$ xclip -selection clipboard -o -t text/html
 <a href="https://stackoverflow.com/users/200540/rkhayrov" title="3017 reputation" class="comment-user">rkhayrov</a>
$ xclip -selection clipboard -o -t UTF8_STRING
 rkhayrov
$ xclip -selection clipboard -o -t TIMESTAMP
684176350

And xclip can also set and own a selection (-i instead of -o).