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Message-ID: <1793833.mheAe9J7Na@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Fri, 30 Mar 2012 20:46:53 +0200
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Subject: Re: Browser Default Colors
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Dr.Kral@nyc.rr.com wrote:
^^^^^^^^^^^^^^^^^^
It is of advantage if you post using your real name here.

> Browser Default Colors

As this is Network News, the subject of the message is already provided by 
the Subject header field value.  You do not have to and you should not 
repeat that value in the message body.  This applies to several other 
message header fields as well.
 
> I would like to get the default colors used by a browser using ECMAScript
> from within the displayed page.
> 
> The function
>    document.getElementById("xxx").style.backgroundColor
> only reports the value set by inline styles or style sheets.

The property would only yield the value as declared in a `style' attribute, 
if present, and it is evidently not a function.

> The functions
>   elem=document.getElementById("xxx")
>   document.defaultView.getComputedStyle(elem, "").backgroundColor

The proper (standards-compliant) property access is

  document.defaultView.getComputedStyle(elem, null).backgroundColor

or

  document.defaultView.getComputedStyle(
    elem, null).getPropertyValue("background-color")

or

  document.defaultView.getComputedStyle(
    elem, null).getPropertyCSSValue("background-color")

whereas the second one should return a primitive value and the third one a 
reference to an object (or `null').

> and
>   document.getElementById("xxx").currentStyle.backgroundColor

There is also

  document.getElementById("xxx").runtimeStyle.backgroundColor

in the MSHTML DOM.

> do better and provide the actual rendered color value -- most of the time.

In which instances have you observed which one to not provide it?

> The value reported for the body and link backgrounds are given as
> transparent or rgba(0,0,0,0) as the display shows the default background
> color from the browser definitions.  Is there a way to go one level
> deeper?

No, you are at the deepest level accessible to script authors already.  If 
you disable all your author stylesheets (using the `disabled' property of 
objects implementing the StyleSheet interface), those are most of the time 
the values that are either the browser defaults or declared in a user 
stylesheet overriding the defaults.

Keep in mind that the `body' element is not the root element of the 
document, so the default background color might be provided by the root 
element instead.  However, authors should declare the background color of 
the `body' element, unless they go with the default text color.  So one 
wonders what you think you will be needing this for.

> While I'm asking, I'll also ask about the link colors.  If one check a
> displayed link element one does get unvisited color reliably but browsers
> (for security reasons) may lie about the visited color.

Correct.  (Your *question* being?)


PointedEars
-- 
> If you get a bunch of authors […] that state the same "best practices"
> in any programming language, then you can bet who is wrong or right...
Not with javascript. Nonsense propagates like wildfire in this field.
  -- Richard Cornford, comp.lang.javascript, 2011-11-14