Fix references to avoid bikeshed warnings

[css-writing-modes] No longer link to css-multicol-1 for the 'column-gap' property,
as the property definition appears in css-align-3 (without ambiguity).

[css-ruby] Refer to 'CSS-FLEXBOX-1' instead of 'CSS3-FLEXBOX-1'

[css-conditional] Avoid indentation 'fatal error'.

[css-box] Disambiguate 'size' in css-box-3: it refers to a width or height measurement, not an infra data structure size.

Author: ericwilligers

css-box-3/Overview.bs

@@ -12,6 +12,9 @@ Abstract: This specification describes the margin and padding properties, which
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Ignored Terms: internal table elements, block layout
 </pre>
+<pre class='link-defaults'>
+spec:css-sizing-3; type:dfn; text:size
+</pre>
 
 Introduction {#intro}
 ============

css-conditional-3/Overview.bs

@@ -651,6 +651,7 @@ partial namespace CSS {
         would return true,
         return <code>true</code>.
       3. Otherwise, return <code>false</code>.
+
     </div>
 </dl>