css property 和 attribute 的区别
css property可以包含下列這些屬性：
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Value: legal values & syntax
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Initial: initial value
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Applies to: elements this property applies to
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Inherited: whether the property is inherited
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Percentages: how percentage values are interpreted - 是否接受百分號作為屬性值
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Media: which media groups the property applies to
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Computed value: how to compute the computed value
屬性值定義的幾種方式：
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Several juxtaposed words mean that all of them must occur, in the given order.
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A bar (|) separates two or more alternatives: exactly one of them must occur.
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A double bar (||) separates two or more options: one or more of them must occur, in any order.
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A double ampersand (&&) separates two or more components, all of which must occur, in any order.
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Brackets ([ ]) are for grouping.
User Agent, 即瀏覽器。
The first declaration on the BODY element sets the font family to “Gill Sans”. If that font is not available, the user agent (often referred to as a “browser”) will use the ‘sans-serif’ font family which is one of five generic font families which all users agents know. Child elements of BODY will inherit the value of the ‘font-family’ property.
Attribute
A value associated with an element, consisting of a name, and an associated (textual) value.
Content
The content associated with an element in the source document. Some elements have no content, in which case they are called empty.
The content of an element may include text, and it may include a number of sub-elements, in which case the element is called the parent of those sub-elements.
Author
An author is a person who writes documents and associated style sheets. An authoring tool is a User Agent that generates style sheets.
css的書寫者。
User
A user is a person who interacts with a user agent to view, hear, or otherwise use a document and its associated style sheet. The user may provide a personal style sheet that encodes personal preferences.
最終用戶
User agent (UA)
A user agent is any program that interprets a document written in the document language and applies associated style sheets according to the terms of this specification. A user agent may display a document, read it aloud, cause it to be printed, convert it to another format, etc.
An HTML user agent is one that supports one or more of the HTML specifications. A user agent that supports XHTML [XHTML], but not HTML is not considered an HTML user agent for the purpose of conformance with this specification.
Property
CSS defines a finite set of parameters, called properties, that direct the rendering of a document.
Each property has a name (e.g., ‘color’, ‘font’, or border’) and a value (e.g., ‘red’, ‘12pt Times’, or ‘dotted’). Properties are attached to various parts of the document and to the page on which the document is to be displayed by the mechanisms of specificity, cascading, and inheritance (see the chapter on Assigning property values, Cascading, and Inheritance).
CSS property和document渲染相關，但attribute無關。
看個attribute的例子：
<html><div><span>1</span><span>2</span></div><div id = "jerry" actions tool="spa"><p>3</p><p>4</p></div></html>percentage property
Percentage values are always relative to another value, for example a length.
Percentage values總是和另一個屬性值相關聯。
Each property that allows percentages also defines the value to which the percentage refers. The value may be that of another property for the same element, a property for an ancestor element, or a value of the formatting context (e.g., the width of a containing block).
比如我指定了一個max-wdith：60%，那么這個60%到底是另一個什么樣的屬性的60%呢？
另一個屬性可能是同一個element的另一個屬性，也可能是其祖先element的某一個屬性，或者是formatting context的value，比如containing block的width.
When a percentage value is set for a property of the root element and the percentage is defined as referring to the inherited value of some property, the resultant value is the percentage times the initial value of that property.
percentage value的繼承問題
Since child elements (generally) inherit the computed values of their parent, in the following example, the children of the P element will inherit a value of 12px for ‘line-height’, not the percentage value (120%):
p { font-size: 10px }p { line-height: 120% } /* 120% of 'font-size' */子節點繼承父節點的property是calculated property，而不是percentage property.
總結
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