Question

How to work out how CSS attributes are inherited

Asked by: tazevans

Firebug can display the css attributes of any element or selector. It can also display (see example below) the attributes inherited from parent elements. Does anyone know how this is done?

p {style.css (line 83)
color:#000000;
font-size:1em;
margin:0.75em 0pt;
}
 
Inherited from div.content
 
#maincol .content {style.css (line 80)
font-size:1.1em;
}
 
Inherited from div#pagewidth
 
#pagewidth {style.css (line 6)
text-align:left;
}
 
Inherited from body#forum.layout-left
 
html, body {style.css (line 1)
color:#000000;
font-family:Verdana,Arial,Helvetica,sans-serif;
font-size:84%;
font-size-adjust:none;
font-style:normal;
font-variant:normal;
font-weight:normal;
line-height:normal;
}
 
Inherited from html.js
 
html, body {style.css (line 1)
color:#000000;
font-family:Verdana,Arial,Helvetica,sans-serif;
font-size:84%;
font-size-adjust:none;
font-style:normal;
font-variant:normal;
font-weight:normal;
line-height:normal;
}

                                  
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Asked On
2008-05-04 at 02:18:28ID23374682
Tags

Javascript

,

Internet Explorer 7, Firefox

Topics

JavaScript

,

Cascading Style Sheets (CSS)

,

Dynamic HTML (DHTML)

Participating Experts
2
Points
250
Comments
2

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Answers

 

by: cwolvesPosted on 2008-05-04 at 11:06:08ID: 21496397

It's done with internal firefox debugging classes that aren't exposed to the client.  There is no simple way to emulate this, but if you -really- want to here's what it involves doing:

function showStyles(obj, hash)
   if(!hash){ hash = {}; }
   // read obj.style, push each attribute into a hash, removing existing ones as being "overwritten"
   // read all classes from all stylesheets and check the xpath of each one against obj
   // for any matched classes, push each attribute into a hash, removing existing ones as being "overwritten"
   if(obj.parentNode){ showStyles(obj.parentNode, hash); }
   return hash;
}

it's fairly complicated and slow, I wouldn't recommend trying it  :-)

 

by: KravimirPosted on 2008-05-04 at 11:11:11ID: 21496412

Some of that functionality is built into DOM Inspector, but you can look at the parts that Firebug does itself.

In lib.js there is a list of properties ("inheritedStyleNames") that can be automatically inherited, so for a given element it looks for such properties in the rules being applied to each ancestor. (That list doesn't include the special print or aural properties.)

The table here (http://www.w3.org/TR/CSS21/propidx.html) shows which properties are inherited by default.

"updateCascadeView" in css.js will also be of interest.


Via http://code.google.com/p/fbug/source/browse/ you can look at the files:

branches/firebug1.2/content/firebug/css.js
branches/firebug1.2/content/firebug/lib.js

20120131-EE-VQP-002

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