What are Cascading Style Sheets?
A Cascading Style Sheet (CSS) is a list of statements (also known as rules) that can assign various rendering properties to HTML elements. Style rules can be specified for a single element occurrence, multiple elements, an entire document, or even multiple documents at once. It is possible to specify many different rules for an element in different locations using different methods. All these rules are collected and merged (known as a "cascading" of styles) when the document is rendered to form a single style rule for each element.
How do I center block-elements with CSS1?
1. By setting the properties margin-left and margin-right to auto and width to some explicit value:
BODY {width: 30em; background: cyan;}
P {width: 22em; margin-left: auto; margin-right: auto}
In this case, the left and right margins will each be four ems wide, since they equally split up the eight ems left over from (30em - 22em). Note that it was not necessary to set an explicit width for the BODY element; it was done here to keep the math clean.
Another example:
TABLE {margin-left: auto; margin-right: auto; width: 400px;}
In most legacy browsers, a table's width is by default determined by its content. In CSS-conformant browsers, the complete width of any element (including tables) defaults to the full width of its parent element's content area. As browser become more conformant, authors will need to be aware of the potential impact on their designs.
If background and color should always be set together, why do they exist as separate properties?
There are several reasons for this. First, style sheets become more legible -- both for humans and machines. The background property is already the most complex property in CSS1 and combining it with color would make it even more complex. Second, color inherits, but background doesn't and this would be a source of confusion.
What is class?
Class is a group of 1) instances of the same element to which an unique style can be attached or 2) instances of different elements to which the same style can be attached.
1) The rule P {color: red} will display red text in all paragraphs. By classifying the selector P different style can be attached to each class allowing the display of some paragraphs in one style and some other paragraphs in another style.
2) A class can also be specified without associating a specific element to it and then attached to any element which is to be styled in accordance with it's declaration. All elements to which a specific class is attached will have the same style.
To classify an element add a period to the selector followed by an unique 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. (Note: in HTML the value of the CLASS attribute can contain more characters). (Note: text between /* and */ are my comments).
CSS
P.name1 {color: red} /* one class of P selector */
P.name2 {color: blue} /* another class of P selector */
.name3 {color: green} /* can be attached to any element */
HTML
<P class=name1>This paragraph will be red</P>
<P class=name2>This paragraph will be blue</P>
<P class=name3>This paragraph will be green</P>
<LI class=name3>This list item will be green</LI>
It is a good practice to name classes according to their function than their appearance; e.g. P.fotnote and not P.green. In CSS1 only one class can be attached to a selector. CSS2 allows attaching more classes, e.g.:
P.name1.name2.name3 {declaration} <P class="name1 name2 name2">This paragraph has three classes attached</P>
What is grouping ?
Grouping is gathering (1) into a comma separated list two or more selectors that share the same style or (2) into a semicolon separated list two or more declarations that are attached to the same selector (2).
1. The selectors LI, P with class name .first and class .footnote share the same style, e.g.:
LI {font-style: italic}
P.first {font-style: italic}
.footnote {font-style: italic}
To reduce the size of style sheets and also save some typing time they can all be grouped in one list.
LI, P.first, .footnote {font-style: italic}
2. The declarations {font-style: italic} and {color: red} can be attached to one selector, e.g.:
H2 {font-style: italic}
H2 {color: red}
and can also be grouped into one list:
H2 {font-style: italic; color: red}
What is external Style Sheet? How to link?
External Style Sheet is a template/document/file containing style information which can be linked with any number of HTML documents. This is a very convenient way of formatting the entire site as well as restyling it by editing just one file. The file is linked with HTML documents via the LINK element inside the HEAD element. Files containing style information must have extension .css, e.g. style.css. <HEAD> <LINK REL=STYLESHEET HREF="style.css" TYPE="text/css"> </HEAD>
Is CSS case sensitive?
Cascading Style Sheets (CSS) is not case sensitive. However, font families, URLs to images, and other direct references with the style sheet may be.
The trick is that if you write a document using an XML declaration and an XHTML doctype, then the CSS class names will be case sensitive for some browsers.
It is a good idea to avoid naming classes where the only difference is the case, for example:
div.myclass { ...}
div.myClass { ... }
If the DOCTYPE or XML declaration is ever removed from your pages, even by mistake, the last instance of the style will be used, regardless of case.
Three selectors: h1, .warning and #footer, what they do ?
An element points at a HTML-tag somewhere on your page. In the example above we want to style the <h1>-tag. Note that using an element like that affects all tags with that name, so using p { margin-left: 100px; } gives all <p>-tags a left-margin.
Using a class is just as simple. When writing .your_class you style all tags with a class with the name “your_class”. In the example above we have .warning which will style e.g. <div class="warning"> and <em class="warning">, that is, any element with the class warning. Classes are used when you want to style just a few of your tags in a way, perhaps you want some of your links red? Add a class to all those links.
You need one more building block: the id. This time you style an element with the attribute “id” set to the id you have chosen. Ids work exactly like classes except for one thing; you can only have one id with a certain name in each of your HTML documents. In the example above we style <div id="footer">. If you look at the example it does make sense: a HTML document may contain several warnings but only one footer. Ids should be used when you want to style just one specific tag.
Using those three building blocks will take you far but when you get to more advanced layouts you might want to combine the building blocks into more advanced selectors. Just to give you two examples of what you can do: em.warning to style only those <em>-tags with the class .warning set. You can also use #footer a to style only the links that are nested inside the tag with id “footer.
Each of the selectors has a set of declarations tied to them. Each declaration has a property, describing what we want to change and a value, what we should change it to. An example: a { color: Blue; font-size: 3em; }. You have the selector a there, so all links in your document will be styled. We have two declarations: color: Blue and font-size: 3em;. Lastly each declaration consists of two parts: the property color and the value Blue.
there is a LOT of things you can style and play with. Additionally (close to) all tags are equal in CSS, so you can set e.g. borders and colors of any element just like you could with a table if you used only HTML.
What are Style Sheets?
Style Sheets are templates, very similar to templates in desktop publishing applications, containing a collection of rules declared to various selectors (elements).
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
'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 <BODY> 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 ...
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.
<HEAD>
<STYLE TYPE="text/css">
<!--
P {text-indent: 10pt}
-->
</STYLE>
</HEAD>
Note: The styling rules are written as a HTML comment, that is, between <!-- and --> to hide the content in browsers without CSS support which would otherwise be displayed.
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}
<P ID=abc123>This and only this element can be identified as abc123 </P>
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
How do I have a background image that isn't tiled?
Specify the background-repeat property as no-repeat. You can also use the background property as a shortcut for specifying multiple background-* properties at once. Here's an example:
BODY {background: #FFF url(watermark.jpg) no-repeat;}
What does \ABCD (and \ABCDE) mean?
CSS allows Unicode characters to be entered by number. For example, if a CLASS value in some Russian document contains Cyrillic letters EL PE (Unicode numbers 041B and 041F) and you want to write a style rule for that class, you can put that letter into the style sheet by writing:
.\041B\041F {font-style: italic;}
This works on all keyboards, so you don't need a Cyrillic keyboard to write CLASS names in Russian or another language that uses that script.
The digits and letters after the backslash (\) are a hexadecimal number. Hexadecimal numbers are made from ordinary digits and the letters A to F (or a to f). Unicode numbers consist of four such digits.
If the number starts with a 0, you may omit it. The above could also be written as:
.\41B\41F {font-style: italic;}
But be careful if the next letter after the three digits is also a digit or a letter a to f! This is OK: .\41B-\41F, since the dash (-) cannot be mistaken for a hexadecimal digit, but .\41B9\41F is only two letters, not three.
Four digits is the maximum, however, so if you write:
.\041B9\041F {font-style: italic;}
Why do style sheets exist?
SGML (of which HTML is a derivative) was meant to be a device-independent method for conveying a document's structural and semantic content (its meaning.) It was never meant to convey physical formatting information. HTML has crossed this line and now contains many elements and attributes which specify visual style and formatting information. One of the main reasons for style sheets is to stop the creation of new HTML physical formatting constructs and once again separate style information from document content.
What are the advantages/disadvantages of the various style methods?
External Style Sheets
Advantages
* Can control styles for multiple documents at once
* Classes can be created for use on multiple HTML element types in many documents
* Selector and grouping methods can be used to apply styles under complex contexts
Disadvantages
* An extra download is required to import style information for each document
* The rendering of the document may be delayed until the external style sheet is loaded
* Becomes slightly unwieldy for small quantities of style definitions
Embedded Style Sheets
Advantages
* Classes can be created for use on multiple tag types in the document
* Selector and grouping methods can be used to apply styles under complex contexts
* No additional downloads necessary to receive style information
Disadvantages
* This method can not control styles for multiple documents at once
Inline Styles
Advantages
* Useful for small quantities of style definitions
* Can override other style specification methods at the local level so only exceptions need to be listed in conjunction with other style methods
Disadvantages
* Does not distance style information from content (a main goal of SGML/HTML)
* Can not control styles for multiple documents at once
* Author can not create or control classes of elements to control multiple element types within the document
* Selector grouping methods can not be used to create complex element addressing scenarios
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