text processing - How to delete all lines before and after the specific strings in the file?

I have played with Powergrep and regular expressions but I don't know how to do that:

I have the text file (source code of webpage) and I want to delete all text (a lot of lines) before the specific string in the code: STRING1 (html tag) and also all the text after another specific string: STRING2. Those strings appears only once in the source code.