safe-printable-inset META value proposal

rfcs/safe-printable-inset.md

@@ -0,0 +1,35 @@
+# RFC #233: Simulate safe printable inset
+
+## Summary
+
+We need a way of testing the [safe-printable-inset](https://github.com/w3c/csswg-drafts/pull/13190/files) property.
+
+## Details
+
+Most printers have a small region along each edge of the paper edges that's not
+reliably printable, usually due to the printer's paper handling mechanism.
+Authors can steer clear of such unprintable areas using the
+`safe-printable-inset` property, which applies in `@page` and `@page` margin
+contexts.
+
+There should be a way for print reftests to test this, by simulating unprintable
+areas.
+
+One rather straight-forward solution would be a META value that sets the width
+of the unprintable area on all four sides. For instance:
+
+`<meta name="safe-printable-inset" content="[inset-specifier]">`
+
+where `inset-specifier` is a numeric value. This values is in centimeters, to
+match the other inputs to webdriver when printing. Using centimeters isn't
+great, since they don't convert nicely into CSS pixels (unlike inches), but
+since everything else is already in centimeters, better follow suit.
+
+Why just one value for all four edges? Although many printers indeed don't
+necessarily have a uniform unprintable area width along each of the four paper
+edges (although many do), so that just providing one value for all is an
+oversimplification of reality, printers may rotate the print output at their own
+discretion. The user agent may therefore not be able to make assumptions about
+which edge (long or short?) will be fed first into the printer, or what
+orientation the sheet of paper has. Therefore using just one value (which should
+represent the larger of the four) seems reasonable.