@meso really, the full PERL regex allow processing cyclic references (usually with limited indirection level), but this is killer ineffective and memory consuming process. and now every browser has to use totally inefective parsers. they're principally ineffective. this is the consequence of sick design of html5. moreover, it allows creating bomb HTML code that will intentionally devour CPU and memory and this is also a security undermine or something like this. needless to say, nobody really even needed that cyclic references in HTML. this was pure stupidity when they implemented that evil bug in HTML.