[css-mediaqueries] Adjust headings hierarchy

Also fixup broken markup

Author: frivoal

mediaqueries-4/Overview.bs

@@ -1727,7 +1727,7 @@ max brightness or low brightness as it relates to displays. As such, the
 determination for '@media/dynamic-range' and '@media/video-dynamic-range' will
 be defined by the user agent.
 
-<h3 id="video-prefixed-features">Video Prefixed Features</h3>
+<h2 id="video-prefixed-features">Video Prefixed Features</h2>
 
     Some user agents, including many TVs, render video and graphics in two
 	separate "planes" (bi-plane) with distinct screen characteristics. A set of
@@ -1750,7 +1750,6 @@ Video Color Display Quality: the 'video-color-gamut' feature</h3>
 	For: @media
 	Type: discrete
 	</pre>
-</h3>
 
 The '@media/video-color-gamut' media feature describes the approximate range of colors
 	that are supported by the UA and output device's video plane.
@@ -1769,7 +1768,6 @@ Video Dynamic Range: the 'video-dynamic-range' feature</h3>
 	For: @media
 	Type: discrete
 	</pre>
-</h3>
 
 '@media/video-dynamic-range' represents the combination of max brightness,
 color depth, and contrast ratio that are supported by the UA and output device's