CSS 面试题与答案

问题 41. What is CSS rule 'ruleset'?

There are two types of CSS rules: ruleset and at-rule. Ruleset identifies selector or selectors and declares style which is to be attached to that selector or selectors. For example P {text-indent: 10pt} is a CSS rule. CSS rulesets consist of two parts: selector, e.g. P and declaration, e.g. {text-indent: 10pt}.

P {text-indent: 10pt} - CSS rule (ruleset)
{text-indent: 10pt} - CSS declaration
text-indent - CSS property
10pt - CSS value

问题 42. 'Fixed' Background?

There is the possibility to use the HTML tag bgproperties="fixed", but that is IE proprietary, and dependent upon the 'background' attribute (deprecated in HTML4).

With CSS, you can declare the background like:

BODY {
font-family : "Trebuchet MS", Verdana, Arial, Helvetica, sans-serif;
background-image: url(images/yourimage.gif);
background-repeat: no-repeat; /*no-tiling background*/
background-position: center;
background-attachment: fixed;
background-color: #hexcolor;
color : #hexcolor;
margin: 10px;
}

that shows a background-image in the center of the element, non-scrolling and non-repeating - in IE or NN6. NN 4.xx gets the non-repeat-part right, but stuffs the picture in the upper left corner and scrolls ...

问题 43. What is embedded style? How to link?

Embedded style is the style attached to one specific document. The style information is specified as a content of the STYLE element inside the HEAD element and will apply to the entire document.

问题 44. What is ID selector?

ID selector is an individually identified (named) selector to which a specific style is declared. Using the ID attribute the declared style can then be associated with one and only one HTML element per document as to differentiate it from all other elements. ID selectors are created by a character # followed by the selector's name. The name can contain characters a-z, A-Z, digits 0-9, period, hyphen, escaped characters, Unicode characters 161-255, as well as any Unicode character as a numeric code, however, they cannot start with a dash or a digit.

#abc123 {color: red; background: black}

This and only this element can be identified as abc123

问题 45. What is contextual selector?

Contextual selector is a selector that addresses specific occurrence of an element. It is a string of individual selectors separated by white space, a search pattern, where only the last element in the pattern is addressed providing it matches the specified context.

TD P CODE {color: red}

The element CODE will be displayed in red but only if it occurs in the context of the element P which must occur in the context of the element TD.

TD P CODE, H1 EM {color: red}

The element CODE will be displayed in red as described above AND the element EM will also be red but only if it occurs in the context of H1

P .footnote {color: red}

Any element with CLASS footnote will be red but only if it occurs in the context of P

P .footnote [lang]{color: red}

Any element with attribute LANG will be red but only if it is classed as "footnote" and occurs in the context of P