[css-values-4] Define that negative ranges are always invalid. #8532

Author: tabatkins

css-values-4/Overview.bs

@@ -2725,6 +2725,10 @@ Resolution Units: the <<resolution>> type and ''&lt;resolution&gt;/dpi'', ''&lt;
 	All <<resolution>> units are <a>compatible</a>,
 	and ''&lt;resolution>/dppx'' is their <a>canonical unit</a>.
 
+	The allowed range of <<resolution>> values
+	<em>always</em> excludes negative values,
+	in addition to any explicit ranges that might be specified.
+
 	<p class="note">Note that due to the 1:96 fixed ratio of CSS ''in'' to CSS ''px'',
 	''1dppx'' is equivalent to ''96dpi''.
 	This corresponds to the default resolution of images displayed in CSS: