[css-writing-modes] Clarify implementation requirements for obsolete writing-mode keywords, add UA style rules that can help in some cases.

Author: fantasai

css-writing-modes/Overview.bs

@@ -886,14 +886,29 @@ Block Flow Direction: the 'writing-mode' property</h3>
      then its comptued <a>inner display type</a> becomes ''flow-root''. [[!CSS-DISPLAY-3]]
   </ul>
 
+  <div class="note" id="embedded-svg-inheritance">
+    <p>As all other inherited CSS properties do,
+    the 'writing-mode' property inherits to SVG elements inlined 
+    (rather than linked) into the source document.
+    This could cause unintentional side effects when, for example,
+    an SVG image designed only for horizontal flow was embedded into a vertical flow document.
+
+    <p>Authors can prevent this from happening by adding the following rule:
+    <div class="example">
+      <pre>svg { writing-mode: initial; }</pre>
+    </div>
+  </div>
+
 <h4 id="svg-writing-mode">
-SVG1.1 'writing-mode' Values</h4>
+Obsolete SVG1.1 'writing-mode' Values</h4>
 
   <p>SVG1.1 [[!SVG11]] defines some additional values: ''lr'',
     ''lr-tb'', ''rl'', ''rl-tb'', ''tb'', and ''tb-rl''.
 
-  <p>These values are <em>deprecated</em> in any context except SVG1 documents.
-    Implementations that wish to support these values in the context of CSS
+  <p>These values are <em>obsolete</em> in any context except SVG1 documents
+    and are therefore <em>optional</em> for non-SVG UAs.
+
+  <p>UAs that wish to support these values in the context of CSS
     must treat them as follows:
 
   <table class="data">
@@ -915,27 +930,31 @@ SVG1.1 'writing-mode' Values</h4>
     specification. The additional ''tb-lr'' value of that revision is
     replaced by ''vertical-lr''.
 
-  <p>In SVG1.1, these values set the <dfn>inline progression direction</dfn>,
+  <p>UAs that only need to support the SVG presentational attribute syntax,
+    but not the CSS syntax, for these values
+    may instead use UA style sheet rules to make the mapping:
+
+  <pre highlight=css>
+    @namespace svg "http://www.w3.org/2000/svg";
+    svg|*[writing-mode=lr], svg|*[writing-mode=lr-tb],
+    svg|*[writing-mode=rl], svg|*[writing-mode=rl-tb] {
+      writing-mode: horizontal-tbl; }
+    svg|*[writing-mode=tb], svg|*[writing-mode=tb-rl] {
+      writing-mode: vertical-rl; }
+  </pre>
+
+  <div class="note">
+    In SVG1.1, these values set the <dfn>inline progression direction</dfn>,
     in other words, the direction the current text position
     advances each time a glyph is added. This is a geometric process that
     happens <em>after</em> bidi reordering, and thus has no effect on the
     interpretation of the 'direction' property (which is independent of
     'writing-mode'). (See <a href="http://www.w3.org/TR/SVG/text.html#RelationshipWithBiDirectionality">Relationship
     with bidirectionality</a>. [[!SVG11]])
-  <p class="note">There are varying interpretations
-    on whether this process causes "writing-mode: rl" to merely shift the
-    text string or reverse the order of all glyphs in the text.</p>
-
-  <div class="note" id="embedded-svg-inheritance">
-    <p>As all other CSS properties do,
-    the 'writing-mode' property inherits to embedeed inline SVG elements.
-    This could cause unintentional side effects when, for example,
-    an SVG designed only for horizontal flow was embedded inline into a vertical flow document.
 
-    <p>Authors can prevent this from happening by adding the following rule:
-    <div class="example">
-      <pre>svg { writing-mode: initial; }</pre>
-    </div>
+    There are varying interpretations
+    on whether this process causes "writing-mode: rl" to merely shift the
+    text string or reverse the order of all glyphs in the text.
   </div>
 
 <h2 id="inline-alignment">