The cascade and inheritance — CSS: selectors, cascade and specificity

The cascade: when several rules compete

"I set color: red and the text is still blue." That is, by far, the number-one question from anyone starting out with CSS. And it's almost never a bug: it's the cascade doing its job. Several rules touching the same property may apply to the same element, and the cascade (the C in CSS) is the algorithm that decides which one wins. The browser orders them, in this order of priority:

Origin and importance. The author's stylesheets (yours) carry more weight than the browser's defaults. A declaration with !important is bumped up a level.
Specificity. With equal origin, the most specific selector wins (we look at this in detail in the next lesson).
Order of appearance. If specificity ties, the last declared rule wins.

p { color: black; }
p { color: blue; }   /* this one wins: same specificity, comes later */

In the example both rules have identical specificity, so order decides: the <p> is painted blue. Quick mental rule: when weights tie, the one you wrote last wins. That's why sometimes just moving a rule a few lines down is enough for it to finally "apply".

Inheritance: properties that travel downwards

Some properties, if not specified, are inherited from the parent element. They are mainly those related to text:

Inherited: color, font-family, font-size, line-height, text-align, visibility...
Not inherited: margin, padding, border, background, width, height, display...

body { color: #333; font-family: system-ui; }
/* A <p> inside <body> inherits color and font-family
   even if it has no rule of its own. */

You can force the behavior with special values:

.box { color: inherit; }    /* inherits explicitly from the parent */
a    { color: inherit; }    /* the link takes the color of the surrounding text */

Inheritance avoids repeating rules: you define the base typography in body once and the whole document respects it except where you override it. Think of it like a family surname: the child carries it without asking, until they decide to change it on purpose.