Add safePrintableInset parameter for printing.

index.html

@@ -11313,6 +11313,13 @@ <h3><dfn>Print Page</dfn></h3>
  <li><p>If <var>shrinkToFit</var> is not a <a>Boolean</a>
  return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
 
+ <li><p>Let <var>safePrintableInset</var> be the result of <a>getting a
+  property with default</a> named "<code>safePrintableInset</code>" and with
+  default <code>0</code> from <var>parameters</var>.
+
+ <li><p>If <var>safePrintableInset</var> is not a <a>Number</a>, or is less than 0,
+ return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
+
  <li><p>Let <var>pageRanges</var> be the result of <a>getting a
   property with default</a> named "<code>pageRanges</code>" from
   <var>parameters</var> with default of an
@@ -11364,6 +11371,13 @@ <h3><dfn>Print Page</dfn></h3>
        page width specified in the content
      </dl>
 
+   <p>The value of <var>safePrintableInset</var> defines the minimum inset along
+   each page edge needed to steer clear of the area that the printer is potentially
+   incapable of marking reliably. Most printers have a small region along each
+   edge of the page sheet which is unprintable, typically due to the printer's
+   paper handling mechanism. The document that is being printed may have CSS rules
+   to adjust page margins based on this. TODO: Link to spec, when it has landed?
+
   <li><p>If <var>pageRanges</var> is not an empty <a>Array</a>,
  Let <var>pages</var> be the result of <a>trying</a> to <a>parse a
  page range</a> with arguments <var>pageRanges</var> and the number of