Power Pages rendering in PowerApps Portals works by injecting a standard header and footer into the page. However, if you create a page template that doesn't use site headers, your web template will be responsible for the rendering from start to end. This gives you full control but may cause you to lose some automatic injections from Power Pages. There are also snippets available, such as the "Header/Bottom" snippet, which allows you to add extra output to the head portion of the page.
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