[css-text-decor-4] Floor thickness at 1 device pixel

Author: fantasai

css-text-decor-4/Overview.bs

@@ -58,6 +58,7 @@ Text Decoration Line Thickness: the 'text-decoration-width' property</h3>
 		<dt><dfn><<length>>
 		<dd>
 			Specifies the thickness of text decoration lines as a length.
+			The UA must floor the actual value at one device pixel.
 			<span class="advisement">
 			Authors are strongly encouraged to use ''em'' units
 			so that the line thickness scales with the font.