I don't think you really want to be messing with editing .dll files.  These are normaly dynamically linked and loaded executables, so editing them is a little bit more than just typing in stuff to replace what is there.  Generally if you edit any code that will be executed, you need to keep the size the same or you will get segment faults and other things when executing the code.
If you still want to do editing of the .DLL file, don't touch anything that does not look like text because it is likely executable code.  Open the file with a hex editor and find the place where the data code that contains the .html stuff is.  If you find it and for every character that is present, replace it with something else, then you should be fine.  If there are extra characters, you will need to leave spaces in their place to keep the size the same and you usually can't add extra characters without more experience on how to modify other areas of the .DLL.