Pretty short which is good.
":lang(ja)" in CSS is interesting if it actually works.
I prefer HTML to be either moved out of the code file or actually moved into the code (using literals instead of string catting) but it's not too bad here.
The two different functions named CleanString worry me.
It uses an unnecessarily long charset declaration instead of <meta charset="utf-8">.
The iconv charset name for tripcodes is "CP932", not "Shift_JIS". (although it's hard to tell the difference)
The page style has poor spacing between some elements.
The spam filter/IP ban lists in text files isn't really sufficient.