im assuming a few things as i write this, #1 you are going to write this app for a corporate setting, #2 the file will not be sitting on the users box, #3 the pages are not ASPs.
I am thinking your gunna have to make a file box that does not show the path.
The html files are going to have to adhere to a strict format regarding placement of the main tags tags.
I suggest you use line input statements putting the lines into an array.
Use an if statement to see if the tag is "<title>" or "<body>" if so get the next lines and keep concatinating the new string into the same array element until the line just read is the corresponding close tags, put the close tag into the next array index.
Feed the "meat" (for lack of better word) into your "editor" for editing by the user.
I suggest a textbox for the title and a separate textbox for the body.
When saving the new text to the existing HTML file make the title array element equal to the title textbox and make the body textbox equal to the body array element, then write the array to the file using print statements inside a loop.
hope this is what you are looking for,
tryp