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display:none and mixed SVG and HTML

From: Brian Birtles <bbirtles@mozilla.com>
Date: Thu, 02 Feb 2012 16:38:27 +0900
Message-ID: <4F2A3D73.2060607@mozilla.com>
To: 'www-svg' <www-svg@w3.org>
Dear all,

I would like to clarify the expected behaviour of display:none with 
regards to mixed SVG and HTML.

In HTML, display:none generally means that for the purpose of rendering 
the element and its children can be ignored (i.e. no frames are generated).

In SVG, however, sometimes content marked as display:none can be 
rendered when referenced elsewhere.[1]

For example:
  - display:none does not apply to filters[2]
  - display:none does not apply to gradients[3]
  - display:none on clipPath has no effect (although it does on child 
elements)[4]

A more detailed test case is available.[5]

I'd like to clarify how this behaves when SVG and HTML are mixed through 
a series of examples.

Example 0, all SVG:

<svg>
   <g style="display:none">
     <pattern id="pattern">
       <rect fill="blue"/>
     </pattern>
   </g>
   <rect fill="url(#pattern)"/>
</svg>

Expected results: blue rect pattern is rendered.

Rationale: "'pattern' elements are available for referencing even when 
the 'display' property on the 'pattern' element or any of its ancestors 
is set to none."[6]

Example 1, HTML in SVG:

<svg>
   <g style="display:none">
     <pattern id="pattern">
       <foreignObject>
         <body>Hi from HTML</body>
       </foreignObject>
     </pattern>
   </g>
   <rect fill="url(#pattern)"/>
</svg>

Expected results: HTML pattern is rendered.

Rationale: The section from the SVG spec quoted above (Example 0) still 
applies here. We first encounter display:none in SVG mode so SVG 
behaviour applies and the pattern's children are rendered.

Example 2, SVG in HTML, display:none on <p>:

<html>
   <p style="display:none">
     <svg>
       <pattern id="pattern">
         <rect fill="blue"/>
       </pattern>
     </svg>
   </p>
   <svg>
     <rect fill="url(#pattern)"/>
   </svg>
</html>

Expected results: No pattern is rendered, i.e. the rectangle is filled 
black.

Rationale: HTML rendering behaviour kicks in here. The user agent should 
be free to optimise rendering of HTML content by ignoring children of 
display:none elements for the purpose of rendering. Feel free to contest 
this, but this is my proposal here.

Example 3, SVG in HTML, display:none on outer <svg>:

<html>
   <p>
     <svg style="display:none">
       <pattern id="pattern">
         <rect fill="blue"/>
       </pattern>
     </svg>
   </p>
   <svg>
     <rect fill="url(#pattern)"/>
   </svg>
</html>

Expected results: No pattern is rendered, i.e. the rectangle is filled 
black.

Rationale: As with example 2, we are still in HTML mode here when we 
encounter the outermost <svg> element. Again, feel free to contest this.

So in summary:

A. When display:none is first encountered on an SVG element other than 
the outermost SVG element in a document, SVG rendering behaviour applies 
and the element and its children may still be referenced and rendered 
elsewhere where permitted by SVG (e.g. patterns, filters etc.)

B. When display:none is first encountered on an HTML element or an 
outermost SVG element in a compound document the whole subtree from that 
element onwards is not available for rendering. Attempts to reference 
content from that subtree will behave as if the target element was not 
found.

The seems to more-or-less correspond with what I've observed in WebKit. 
For Gecko this is being tracked in bug 376027.[7]

Your feedback would be very much appreciated! Perhaps there are other 
permutations worth clarifying?

Best regards,

Brian Birtles

[1] http://www.w3.org/TR/SVG11/painting.html#DisplayProperty, "The 
‘display’ property only affects the direct rendering of a given element, 
whereas it does not prevent elements from being referenced by other 
elements."
[2] http://www.w3.org/TR/SVG11/filters.html#FilterElement
[3] http://www.w3.org/TR/SVG11/pservers.html#LinearGradientElement
[4] http://www.w3.org/TR/SVG11/masking.html#ClipPathElement
[5] https://bug376027.bugzilla.mozilla.org/attachment.cgi?id=593735
[6] http://www.w3.org/TR/SVG11/pservers.html#PatternElement
[7] https://bugzilla.mozilla.org/show_bug.cgi?id=376027
Received on Thursday, 2 February 2012 07:38:59 GMT

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