Selectors, declarations, and the cascade

What you will learn

• Explain selectors in CSS using the correct mental model
• Trace a focused CSS example and predict its result before execution
• Recognize a boundary case involving declarations and handle it deliberately

The concept

Selectors, declarations, and the cascade is a defining part of practical CSS work. Start by identifying the data or state involved, then trace the operation that changes or interprets it. Pay attention to the rules CSS applies at this boundary, because those rules explain both the useful behavior and the common failure modes. This lesson keeps the example deliberately small, then connects it to specificity, inheritance, and source order so the ideas form a coherent progression rather than a list of isolated syntax facts.

Example

This example is intentionally small so you can trace every line before adapting it.

body {
  font-family: system-ui;
  color: #17352b;
}

Read it step by step

1. 1
   Locate the idea

   Identify where selectors appears in the CSS example and name the data it operates on.

2. 2
   Trace the rule

   Trace the relevant CSS rule one operation at a time, recording any state, type, or control-flow change.

3. 3
   Test a boundary

   Change one input or boundary condition, predict the result, and compare that prediction with the documented outcome.

Common mistakes

Try it yourself