Small issue I encountered while trying to get a full background urxvt working:
If you want Transparency and BorderLess, you should not use transparent but instead background and set it to rgba:0000/0000/0000/0000 Now you get full transparency while having no borders. At least I had the problem that if I used transparent that if used in conjunction with "borderless" the background image that is emulated was not correctly positioned. That works perfectly now (but I think there needs to be some graphical support or something for the rgba option to work).