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问题:

For this question I'm using the following markup:

<body>
    <p>1</p> <!-- Paragraph 1 -->
    <p>2</p> <!-- Paragraph 2 -->
    <p>3</p> <!-- Paragraph 3 -->
</body>

From the Selectors Level 3 specification, the following selector rules apply:

*        any element
E + F    an F element immediately preceded by an E element
E ~ F    an F element preceded by an E element

Based on this, the following should occur:

body + * { } /* Selects nothing as no elements precede body */
body ~ * { } /* As above. */
p + *    { } /* Selects Paragraph 2 and Paragraph 3 as these are preceded by p */
p ~ *    { } /* As above. */
* + *    { } /* As above. */
* ~ *    { } /* As above. */

False!

* + * and * ~ * are somehow able to select Paragraph 1 along with 2 and 3! Paragraph 1 isn't preceded by anything, so should be impossible to access:

body + * { background: #000; }
body ~ * { background: #000; }
p ~ *    { color: #f00; }
p + *    { font-weight: bold; }
* + *    { text-decoration: underline; }
* ~ *    { font-style: italic; }

Result:

As you can see, paragraph 1 isn't preceded by the body or a phantom p, yet it is apparently preceded by something. It should have no custom styling applied to it at all, yet is somehow affected by those last two selectors. What is the logic behind this?

JSFiddle example.

回答1:

* + * Styles any element that is an immediate sibling of any element starting from the document root - Since the <head> is actually an immediate preceding sibling of the body (despite not being visible in your code) this selector targets the body and the last two paragraphs, since the first paragraph isn't immediately following another sibling element. All three paragraphs happened to be underlined due to the nature of text-decoration on block-level descendants in the normal flow.

* ~ * This is basically the same thing as above, except using the general sibling combinator .. it styles downstream sibling element(s) that appear after the <head> regardless of whether they're immediate siblings or not. Since the <body> is the only sibling, this has the same effect as the above selector. The first paragraph is italicized due to inheritance.

p ~ * selects a sibling element that is following a <p> which in your example is the last two paragraphs. p + * styles any element that is immediate sibling of a paragraph, which would also be the last two <p> elements.

回答2:

They are not applied to the first paragraph actually. To demonstrate this, let's change the stylesheet a bit:

* + *    { border-right: solid red }
* ~ *    { border-left: solid black; }

fiddle

They are both applied to the "body" element which is in fact preceded by "head".

回答3:

That's an error in your test case. As you would expect, none of the selectors match the first paragraph, but the styling from the body cascades to the paragraphs!

Both * + * and * ~ * match body as it is preceded by a head tag. Thus, it receives text-decoration:underline and font-style:italic. That explains why all of the paragraphs are both underlined and italicised.