Overriding combined css classes - css

Question

There are numerous questions about the order of loading CSS files and overriding classes, but after reading them I still have something I can't figure out.
There are two CSS files:
<link rel="stylesheet" href="standard.css" type="text/css">
<link rel="stylesheet" href="override.css" type="text/css">
loaded in this order (I checked that).
The HTML looks like this:
<div class="div_D1 ov_D1">
<div class="div_D2 ov_D2">
<div class="div_D3 ov_D3">
blablah
</div>
</div>
</div>
Standard.css contains:
.div_D1{
background: white;
}
.div_D2{
height: 10px;
}
.div_D1 .div_D3{
padding-left: 20px;
}
Override.css contains:
.ov_D1{
background: red;
}
.ov_D2{
height: 50px;
}
.ov_D3{
padding-left: 0px;
}
.ov_D1 and .ov_D2 are applied correctly: the background of .div_D1 is red, the height of .div_D2 is 50px.
.ov_D3 on the other hand does not behave as I expected. If I look at the order the rules are applied, the browser first applies .ov_D3, and then .div_D1 .div_D3, leaving me with an unwanted padding of 20px.
If however I change the class selector in Override.css to
.div_D1 .ov_D3 it does remove the padding.
Also changing the css to
.ov_D3{
padding-left: 0px; !important
}
does the trick. So there are solutions, I only can't understand why with a single selector the order of loading is respected, and with multiple selectors it is not.

Answer 1

This is called specificity of a Selectors. From the book Beginning CSS: Cascading Style Sheets for Web Design, Third Edition by Ian Pouncey and Richard York:
In addition to style sheet precedence, an order of precedence exists for the selectors contained in each style sheet.
This precedence is determined by how specific the selector is.
For example, an ID selector is the most specific,
and the universal selector is the most general. Between these, the
specificity of a selector is calculated using the following formula:
Count 1 if the styles are applied from the (X)HTML style attribute, and 0 otherwise; this becomes variable a.
Count the number of ID attributes in the selector; the sum is variable b.
Count the number of attributes, pseudo-classes, and class names in a selector; the sum is variable c.
Count the number of element names in the selector; this is variable d.
Ignore pseudo-elements.
Now take the four values and put them together in groups of four.
For Example:
Selector : div.someclass.someother
Selector Type : Element Name + Class Name + Class Name
specificity:
0,0,2,1, (a = 0, b = 0,
c = 2, d = 1)

Answer 2

In CSS, there are rules for specificity (quoted from MDN):
The following list of selectors is by increasing specificity:
Universal selectors
Type selectors
Class selectors
Attributes selectors
Pseudo-classes
ID selectors
Inline style
Since you have added specificity to your Selector you weren't able to override by normal CSS class selector.
So your Code
.div_D1 .div_D3is more specific than.div_D3and less specific than.div_D3.ov_D3.
JS Fiddle

Answer 3

As per MDN CSS selectors have rules called 'Specificity' which determine their order of precedence. The more specific a rule is, the greater it's priority regardless of position within a/some stylesheet(s).
A rule such as .class-1 .class-3 has a specificity (it's more specific) higher than .class-3 and takes precedence, as such the less-specific rule cannot override it without the use of !important which negates all other specificity rules. Using the higher specificity rule only takes place with conflicting styles, however.
So, you have set the rule:
.div_D1 .div_D3 { }
The above rule is more specific than:
.ov_D3 { }
Even though they target the same element the rule with the higher specificity takes precedence. You can fix this in your JS Fiddle by prepending the appropriate class structure as defined above.
So, .ov_D3 becomes either:
.div_D1 .ov_D3
or
.ov_D1 .ov_D3
Example here: JS Fiddle