You can't achieve that without having the triangle as a separate object. You can't position background outside the element.
So just put the triangle next to the black bar, have them "float:left;".
To make sure the triangle doesn't go outside the whole progress bar when loading reaches end (provided you want the black bar to go till the end), use "overflow: hidden;" on the progress bar container and theoretically it should work (not tested).
Leave a comment on jurchiks's reply
Change topic type
Link this topic
Provide the permalink of a topic that is related to this topic