It must be some sort of templating language... that is: the server parses the html and replaces anything like "<?pp:...?>" with the results of either executing some code or evaluating some expression or variable. I can't say that I actually know what template engine it is that uses that syntax, though.
Ones that I am familiar with, though are often quite similar. Typical examples being things like "<? ... ?>", "<% ... %>", "<%= ... %>", "[- ... -]", "[+ ... +]", and so on and so on.
What you should be looking for is which handler is being used to handle these pages. You can probably find that in your httpd.conf.
Not an editor command: Wq