[css-2022] Add core definitions for CSS w3c#3337

Author: fantasai

css-2022/Overview.bs

@@ -50,6 +50,54 @@ Introduction {#intro}
 	the CSS Working Group has chosen to publish this profile
 	to define the current scope and state of Cascading Style Sheets as of 2022.
 
+<h3 id="css-glossary">
+What is CSS?</h3>
+
+	<dl export>
+		<dt><dfn lt="CSS | Cascading Style Sheets">Cascading Style Sheets (CSS)</dfn>
+		<dd>
+			CSS is a language for writing [=style sheets=],
+			and is designed to describe the rendering of structured documents
+			(such as HTML and XML)
+			on a variety of media.
+			CSS is used to describe the presentation of a [=source document=],
+			and usually does not change the underlying semantics
+			expressed by its [=document language=].
+
+		<dt><dfn>Style sheet</dfn>
+		<dd>
+			A set of rules that specify the presentation of a document.
+			Style sheets are written by an [=Author=],
+			and interpreted by a [=User Agent=],
+			to present the document to the [=User=].
+
+		<dt><dfn>Source document</dfn>
+		<dd>
+			The document to which one or more style sheets apply.
+			A source document’s structure and semantics are encoded
+			using a [=document language=] (e.g., HTML, XHTML, or SVG).
+
+		<dt><dfn>Author</dfn>
+		<dd>
+			An author is a person who writes documents and associated style sheets.
+			An <dfn>authoring tool</dfn> is a [=User Agent=] that generates style sheets.
+
+		<dt><dfn>User</dfn>
+		<dd>
+			A user is a person who interacts with a user agent
+			to view, hear, or otherwise use the document.
+
+		<dt><dfn lt="user agent | UA">User Agent (UA)</dfn>
+		<dd>
+			A user agent is any program that interprets a document
+			and its associated [=style sheets=].
+			A [=user agent=] may display a document, read it aloud,
+			cause it to be printed, convert it to another format, etc.
+			For the purposes of the CSS specifications,
+			a [=User Agent=] is one that supports and interprets [=Cascading Style Sheets=]
+			as defined in these specifications.
+	</dl>
+
 <h3 id="w3c-process">
 Background: The W3C Process and CSS</h3>