RFC 173 - Use @page for print refests (#173)

rfcs/page-size.md

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+# RFC 173 - `@page` Rules in Print Tests
+
+[RFC 41](https://github.com/web-platform-tests/rfcs/blob/master/rfcs/print_test.md) added support for print reftests. It specified a fixed page size of "5 inches by 3 inches with 0.5 inch margins on all sides".
+
+CSS allows modifying the size and margins of paginated media using an [`@page` rule](https://developer.mozilla.org/en-US/docs/Web/CSS/@page). Per the letter of the previous RFC these rules would be ignored for wpt tests, making the rule itself impossible to test.
+
+To enable testing different paper sizes and margins, this RFC amends the print reftest support, so that where an `@page` rule is specififed in a test or reference file, it overrides the default paper size when generating the paginated rendering of the document.