Терминология

author
the author of an HTML document
block-level element
an element which has a line break before and after (e.g. 'H1' in HTML)
canvas
the part of the UA's drawing surface onto which documents are rendered
contextual selector
a selector that matches elements based on their position in the document structure. A contextual selector consists of several simple selectors. E.g., the contextual selector 'H1.initial B' consists of two simple selectors, 'H1.initial' and 'B'.
CSS
Cascading Style Sheets
CSS1
Cascading Style Sheets, level 1. This document defines CSS1 which is a simple style sheet mechanism for the web.
CSS1 core features
the part of CSS1 that is required in all CSS1 compliant UAs
CSS1 advanced features
features that are described in this specification but labeled as not among the CSS1 core features
declaration
a property (e.g. 'font-size') and a corresponding value (e.g. '12pt')
designer
the designer of a style sheet
document
HTML document
element
HTML element
fictional tag sequence
a tool for describing the behavior of pseudo-classes and pseudo-elements
font size
The size for which a font is designed. Typically, the size of a font is approximately equal to the distance from the bottom of the lowest letter with a descender to the top of the tallest letter with an ascender and (optionally) with a diacritical mark.
HTML
Hypertext Markup Language [2], an application of SGML.
HTML extension
Markup introduced by UA vendors, most often to support certain visual styles. The "FONT", "CENTER" and "BLINK" elements are examples of HTML extensions, as are the "BGCOLOR" attribute. One of the goals of CSS is to provide an alternative to HTML extensions.
inline element
an element which does not have a line break before and after (e.g. 'STRONG' in HTML)
intrinsic dimensions
the width and height as defined by the element itself, not imposed by the surroundings. In this specification we assume that all replaced elements -- and only replaced elements -- come with intrinsic dimensions.
pseudo-element
pseudo-elements are used in CSS selectors to address typographical items (e.g. the first line of an element) rather than structural elements.
pseudo-class
pseudo-classes are used in CSS selectors to allow information external to the HTML source (e.g. the fact that an anchor has been visited or not) to classify elements.
property
a stylistic parameter that can be influenced through CSS. This specification defines a list of properties and their corresponding values.
reader
the person for whom the document is rendered
replaced element
an element that the CSS formatter only knows the intrinsic dimensions of. In HTML, 'IMG', 'INPUT', 'TEXTAREA', 'SELECT' and 'OBJECT' elements can be examples of replaced elements. E.g., the content of the IMG element is often replaced by the image that the SRC attribute points to. CSS1 does not define how the intrinsic dimensions are found.
rule
a declaration (e.g. 'font-family: helvetica') and its selector (e.g. 'H1')
selector
a string that identifies what elements the corresponding rule applies to. A selector can either be a simple selector (e.g. 'H1') or a contextual selector (e.g. 'H1 B') which consists of several simple selectors.
SGML
Standard Generalized Markup Language [3], of which HTML is an application
simple selector
a selector that matches elements based on their type or attributes, e.g 'H1.initial'
style sheet
a collection of rules
tag
HTML tag, e.g. 'H1'.
UA
User Agent, often a "web browser" or "web client"
user
synonymous with "reader"
weight
the priority of a rule

In the text of this specification, single quotes ('...') denote HTML and CSS excerpts.