The box model

The browser treats every element as a box: content sits in the center, followed by padding, border, and margin. With the default content-box, a declared width covers only content. border-box includes padding and border in that width.

Read a rule first

A CSS declaration uses property: value;. The semicolon ends one declaration, and braces group declarations for the same selector. The browser reads rules from top to bottom. When several rules match one element, the cascade considers their source, selector specificity, and order.

<div class="box">Inspect my layers</div>

.box {
  width: 240px;
  padding: 24px;
  border: 8px solid #0ea5e9;
  margin: 20px;
  background: #e0f2fe;
}

Change one value at a time and watch the text, color, size, or position. This connects each declaration to a visible result. Browser developer tools show the result of the same matching process.

A useful checking order

• Confirm that the selector matches the intended HTML element.
• Check the property name, colon, value, and semicolon.
• Test narrow space, longer text, and keyboard use.

A common source of confusion

CSS changes presentation and layout; it does not change the meaning of the HTML. Keep meaningful HTML elements and style them, rather than choosing the wrong element only for its default appearance. Readability comes before decoration.

<div class="box">Inspect my layers</div>