CSS INHERITANCE

Let’s start
with the
document
tree

Before we explore inheritance, we
need to understand the
document tree.

Document trees are made from
HTML elements.

The document tree is just like
your family tree.

An ancestor refers to any
element that is connected but
further up the document tree.

Ancestor

A descendant refers to any
element that is connected but
lower down the document tree.

Descendant

Descendants

A parent is an element that is
connected & directly above an
element in the document tree.

Parent

A child is an element that is
connected & directly below an
element in the document tree.

Child

A sibling is an element that
shares the same parent as
another element.

Parent

Siblings

We also need to understand the
basics of CSS rules before
exploring inheritance.

CSS rules tell browsers
how to render specific elements
on an HTML page.

CSS rules are made up of
five components.

The selector "selects" the
elements on an HTML page that
are affected by the rule.

p { color: red; }

Selector

The declaration block is a
container that consists of
anything between (and including)
the brackets.

p { color: red; }

Declaration block

The declaration tells a browser
how to render any element on a
page that is selected.

p { color: red; }

Declaration

The property is the aspect of that
element that you are choosing to
style.

p { color: red; }

Property

The value is the exact style you
wish to set for the property.

p { color: red; }

Value

Now…
what is
inheritance?

Inheritance is where specific CSS
properties are passed down to
descendant elements.

To see inheritance in action, we
will use the HTML code below:


Lorem ipsum dolor
sit amet consect etuer.

Note that the element sits
inside the element.

We will also use this CSS code.
Note that the element has
not been specified.

p { color: red; }

In a browser, the and 
elements will both be colored
red.

 element

But why is the element
colored red when it has not been
styled?

Because the element has
inherited the color property from
the element.

Inheritance is designed to make
it easier for authors.

Otherwise we would need to
specify properties for all

p { color: red; }
p em { color: red; }

CSS files would be much larger
in size, harder to create and
maintain as well as slower to
download.

Are all CSS
properties
inherited?

No. All CSS properties are
not inherited!

If every CSS property was
inherited, it would make things
much harder for authors.

Authors would have to turn off
unwanted CSS properties for

For example, what would happen
if the border property was
inherited by default…

and we then applied a border to
the element?

p { border: 1px solid red; }

The inside the would
also have a red border.

 element

Luckily, borders are not
inherited, so the would not
have a red border.

 element

Generally speaking, only
properties that make our job
easier are inherited!

So, which
properties are
inherited?

The following CSS properties are
inherited…

azimuth, border-collapse, border-spacing,
caption-side, color, cursor, direction, elevation,
empty-cells, font-family, font-size, font-style,
font-variant, font-weight, font, letter-spacing,
line-height, list-style-image, list-style-position,
list-style-type, list-style, orphans, pitch-range,
pitch, quotes, richness, speak-header, speak-
numeral, speak-punctuation, speak, speech-
rate, stress, text-align, text-indent, text-
transform, visibility, voice-family, volume, white-
space, widows, word-spacing

Yikes! That is a lot of properties.

To simply things, let’s take a look
at some of the
key groups of properties.

Text-related properties that are
inherited:

List-related properties that are
inherited:

And, importantly, the
color property is inherited:

The simple answer is “yes”.
However, font-size is inherited in
a different way to many other
properties.

Rather than the actual value
being inherited, the calculated
value is inherited.

Before explaining how font-size
inheritance works, we need to
look at why
font-size is not directly
inherited.

Let’s start with the
same sample of HTML code we
used earlier:

As before the 
sits inside the .

Now, a font-size is applied to the
 element only. The has
not been specified.

p { font-size: 80%; }

If the font-size value of 80% were
to be inherited, the
 would be sized to 80%
of the element…

and the rendered document
would look like this:

 element

However, this is not the case!
The is the same size as the
.

 element

So how does inheritance work for
font-size?

Let’s look at three examples
in action.

We will use the same HTML
code as before:

Which produces the same
document tree as before.

The element has been given
a font-size of 14px.
Note: pixels are not recommended for sizing fonts due to accessibility
issues associated with older browsers such as IE5 and IE6.

p { font-size: 14px; }

This pixel value (14px) overrides
the browsers default font-size
value (approx 16px). This new
value is inherited by
descendants.

So, the element inherits the
14px value.

element value calcuated value

default font size approx 16px

<body> unspecified approx 16px

 14px 14px

 unspecified inherited value = 14px

a font-size of 85%.

p { font-size: 85%; }

The browsers default font-size
(16px) and the percentage value
(85%) are used to create a
calculated value (16px x 85% =
13.6px). This calculated value is
inherited by descendants.

13.6px calculated value.




 85% 16px x 85% = 13.6px

 unspecified inherited value = 13.6px

a font-size of .85em.
Note: Avoid using EMs for font-size values under 1em as IE5 renders these
values in pixels instead of EMs (.8em is rendered as 8px).

p { font-size: .85em; }

(16px) and the EM value (.85em)
are used to create a calculated
value (16px x .85em = 13.6px).
This calculated value is
inherited by descendants.

13.6px calculated value.




 .85em 16px x .85em = 13.6px

Those examples were too simple.
What about more complex
examples using different
elements?

All elements have been specified
using percentage values.

body { font-size: 85%; }
h1 { font-size: 200%; }
h2 { font-size: 150%; }

(16px) and the body percentage
value (85%) are used to create a
calculated value (16px x 85% =
13.6px). This calculated value is
inherited by descendants unless
new values are specified.

The font-size inheritance in
action
element value calculated font-size


<body> 85% 16px x 85% = 13.6px

<h1> 200% inherited value 13.6px x 200% = 27.2px

<h2> 150% inherited value 13.6px x 150% = 20.4px

 unspecified inherited value = 13.6px

Using inheritance
for efficiency

Authors can use inheritance to
write efficient CSS.

For example, you can set the
color, font-size and font-family on
the body element.

body {
color: #222;
font-family: arial,
helvetica, sans-serif;
font-size: 90%;
}

These properties will be inherited
by all descendant elements.

You can then override the
properties as needed, specifying
new color values...

body {
color: #222;
font-family: arial,
font-size: 90%;
}

h1, h2, h3 { color: green; }
h4, h5, h6 { color: black; }

body {
color: #222;
font-family: arial,
font-size: 90%;
}


h1, h2, h3, h4, h5, h6 {
font-family: georgia,
times, serif;
}

and new font-size values as
needed.

}


h1, h2, h3, h4, h5, h6 {
font-family: georgia,
times, serif;
}

h1 { font-size: 200%; }
h2 { font-size: 150%; }
h3 { font-size: 125%; }
#footer { font-size: 90%; }

Now, go forth and
inherit the world!

CSS INHERITANCE

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CSS INHERITANCE