Add an 'unusual elements' appendix, to help with display:contents.

Author: tabatkins

css-display/Overview.bs

@@ -912,6 +912,152 @@ Appendix A: Glossary</h2>
 
 	See [[!CSS2]] <a href="https://www.w3.org/TR/CSS2/visuren.html">Chapter 9</a> for a fuller definition of these terms.
 
+<h2 class="no-num non-normative" id="unbox">
+Appendix B: Effects of ''display: contents'' on Unusual Elements</h2>
+
+	Some elements aren't rendered purely by CSS box concepts;
+	for example, replaced elements (such as <{img}>),
+	many form controls (such as <{input}>),
+	and SVG elements.
+
+	This appendix defines how they interact with ''display: contents''.
+
+HTML Elements {#unbox-html}
+-----------------------------
+
+	: <{br}>
+	: <{wbr}>
+	::
+		Issue: Ask the WG.
+		Probably should just treat as ''display: none'',
+		because they normally don't have contents,
+		but theoretically they could be treated as "containing" a line feed or zwnj, respectively.
+
+	: <{button}>
+	::
+		Issue: Ask the WG.
+		<{button}> is rendered via standard CSS,
+		but it's often a form control,
+		and looks identical to <code>&lt;input type=button></code>,
+		which is *definitely* in the "semi-replaced" camp
+		that gets ''display: none''.
+		Which should it be consistent with?
+
+	: <{meter}>
+	: <{progress}>
+	::
+		Issue: Ask the WG.
+		These elements are probably replaced elements,
+		but they do have reasonable contents inside of them.
+
+	: <{applet}>
+	: <{canvas}>
+	: <{embed}>
+	: <{object}>
+	::
+		When acting as a [=replaced element=],
+		''display: contents'' behaves as ''display: none''.
+
+		Issue: When non-replaced and displaying their contents,
+		should we treat as a normal element
+		and just strip the wrapper?
+
+	: <{audio}>
+	: <{iframe}>
+	: <{img}>
+	: <{video}>
+	::
+		''display: contents'' behaves as ''display: none''.
+
+	: <{frame}>
+	: <{frameset}>
+	::
+		''display: contents'' behaves as ''display: none''.
+
+	: <{input}>
+	: <{textarea}>
+	: <{select}>
+	::
+		''display: contents'' behaves as ''display: none''.
+
+	: <{legend}>
+	::
+		Per HTML, a <{legend}> with ''display: contents'' is not a [=rendered legend=],
+		so it does not have magical display behavior.
+		(Thus, it reacts to ''display: contents'' normally.)
+
+	: <{details}>
+	: <{fieldset}>
+	::
+		These elements don't have any special behavior;
+		''display: contents'' simply removes their [=principal box=],
+		and their contents render as normal.
+
+	: any other element
+	::
+		Behaves as normal for ''display: contents''.
+
+SVG Elements {#unbox-svg}
+---------------------------
+
+<div class=issue>
+	Discussed in <a href="https://github.com/w3c/svgwg/issues/305">GitHub</a>.
+
+	SVG can be broken down into the following categories of elements:
+
+	: Root <{svg}>
+	::
+		Establishes the SVG formatting context.
+		Three options:
+
+		1. Treat as ''display: none''. (Contents can't usefully be hoisted.)
+		2. Strip the element,
+			but generate an anonymous SVG rendering context around contiguous SVG contents.
+			(In other words, internal SVG elements auto-wrap themselves,
+			like internal table elements do.)
+		3. Strip the element,
+			treat internal SVG elements similar to unknown HTML elements
+			(render as inline by default).
+
+	: Nested <{svg}>
+	: <{g}>
+	: <{use}> (treated as a Shadow DOM container)
+	::
+		Basically just a wrapping element,
+		could reasonably handle ''display: contents'' normally.
+
+	: Shape elements
+	::
+		Core rendering elements,
+		and usually empty.
+		(Contents, if present, are completely ignored.)
+		Could either strip normally and hoist contents,
+		or just treat as ''display: none''.
+
+	: <{text}>
+		Renders contained text,
+		but text doesn't display without a <{text}> wrapper.
+		Therefore, should probably ''display: none''.
+
+	: <{tspan}>
+		Like a <{span}>,
+		can probably usefully be stripped and have the contents hoisted.
+
+	: <{defs}>
+	:: Could theoretically be stripped and display its contents,
+		but that has negative usefulness,
+		so should probably just continue to not display contents.
+		(In other words, continue to ignore 'display' and suppress all boxes, as normal.)
+
+	: all other elements
+	::
+		Don't generate boxes anyway,
+		so they ignore 'display' entirely.
+</div>
+
+
+
+
 <h2 class=no-num id="acknowledgments">
 Acknowledgments</h2>