What does the "+" (plus sign) CSS selector mean?

For example:

p + p {
  /* Some declarations */

}

I don't know what the + means. What's the difference between this and just defining a style for p without + p?

Answer

See adjacent selectors on W3.org.

In this case, the selector means that the style applies only to paragraphs directly following another paragraph.

A plain p selector would apply the style to every paragraph in the page.

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This will only work on IE7 or above. In IE6, the style will not be applied to any elements. This also goes for the > combinator, by the way.

See also Microsoft's overview for CSS compatibility in Internet Explorer.