You could try using includes to make code maintenance easier:
in .ASP your index page could look something like this:
or with .PHP
<!-- #include virtual ="head.inc" -->
<!-- end head.inc -->
<!-- #include virtual ="nav.inc" -->
<!-- end nav.inc -->
<!-- #include virtual ="main-content.inc" -->
<!-- end main-content.inc -->
<!-- #include virtual ="sidebar.inc" -->
<!-- end sidebar.inc -->
<!-- #include virtual ="footer.inc" -->
<!-- end footer.inc -->
This way you are only editing the the seperate include files.
<?php include("head.php"); ?>
<?php include("nav.php"); ?>
So instead of an 'insert content here' comment, you could just use an include call right in your html page, like so:
Hope all that makes sense
<?php include("main-content.php"); ?>
Actually, this is quite an interesting topic: a text editor that allows you to toggle the visibility of code blocks would be a great idea......I wonder if there is one?
Other than that, I'd give a more advanced text editor, like notepad plus mentioned above, or, my favourite: SciTE which at least colour-code different types of code.