property-value-space-backslash-0/length: Add compat summary

property-value-space-backslash-0/length/index.html

@@ -144,7 +144,7 @@ <h2>How it works</h2>
       </p>
       <p>That leaves Case 3, which will lead to the &#x2400; itself and any content after it being ignored since the &#x2400; will terminate the string prematurely, leaving (in the case of this hack) only a valid length (and some permissible trailing whitespace) as the property's value.</p>
 
-      <p><!--FIXME: SUMMARIZE COMPAT--></p>
+      <p>This hack is known to be compatible with IE8-10. It is known to <em>not</em> be compatible with IE11, in contrast to <a href="../../property-value-backslash-0/length/">the spaceless counterpart</a> of this hack.</p>
 
       <h2>Affected browsers</h2>
       <p>Check out the links for corroborating screenshots.</p>