CSS Properties and Standards

CSS is organized as a standards family, not as a single versioned language. The specification evolves in levels and modules, while browsers implement a moving subset of those documents.

Levels And Modules #

The W3C standardized CSS as a level-based system from the start. CSS 1, CSS 2, and later levels were not just minor revisions; each level changed scope and structure enough that older documents became partially obsolete.

Modern CSS is described through modules such as:

• Colors;
• Backgrounds and Borders;
• Flexbox;
• Grid;
• Values and Units;
• Media Queries.

That modular design lets specifications evolve independently, thus browsers can ship features incrementally.

Property Values #

CSS properties operate on several value stages:

• specified value: the value written in the stylesheet;
• computed value: the resolved value after inheritance and relative units are processed;
• used value: the value after layout constraints are applied.

Inheritance follows computed values, thus a child does not inherit a raw percentage when the parent has already resolved it to an absolute length.

For example:

.box {
  width: 80%;
}

If the parent width is 1000px, the computed width becomes 800px.

display #

The display property controls both outer and inner layout behavior.

<display> = <display-outside> <display-inside> <display-listitem>

The practical meaning is:

• outer display controls how the box participates in normal flow;
• inner display controls how children are laid out.

That is why display is not just a cosmetic property. It changes the formatting context.

Length Units #

Absolute and relative units solve different problems.

Common absolute units:

• px
• cm
• mm
• in
• pt
• pc

Common relative units:

• em
• ex
• ch
• cap
• lh
• ic

Relative units are useful because they scale with font metrics and writing mode.

Standards Documents #

The key reference documents are:

• the CSS specification index;
• CSS snapshots, which summarize the current state of the platform;
• individual module drafts, which define feature families.

That structure matters because the word “CSS standard” usually means a set of living documents, not a single monolithic manual.

Practical Rule #

When writing CSS, keep three layers separate:

1. property syntax;
2. computed layout behavior;
3. the standard module that defines the property.

That separation makes it easier to debug browser behavior and to track which parts are stable, experimental, or obsolete.