The phpBB3 templates are formatted with tab and end-of-line (EOL) characters to make them more readable. However, those characters are not removed before the page is sent to the browser, even though they are unnecessary to render the page. If you want to see the difference it can make, go to the index/home page on your own phpBB3 board and click the "View Source" option on your browser. Note all the whitespace garbage that has been sent to your browser. Now return to this board and click the "View Source" on the index/home page here. You'll notice a dramatic difference!
The following modification will show you how to remove those characters before phpBB3 sends a page to the browser.
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