W3C CSS2 Cascading Style Sheets, level 2 - instructional media + ...

Document language
The encoding language of the source document (e.g., HTML or an XML
application).
Element
(An SGML term, see [ISO8879].) The primary syntactic constructs of the
document language. Most CSS style sheet rules use the names of these
elements (such as "P", "TABLE", and "OL" for HTML) to specify rendering
information for them.
Replaced element
An element for which the CSS formatter knows only the intrinsic dimensions.
In HTML, IMG, INPUT, TEXTAREA, SELECT, and OBJECT elements can
be examples of replaced elements. For example, the content of the IMG
element is often replaced by the image that the "src" attribute designates.
CSS does not define how the intrinsic dimensions are found.
Intrinsic dimensions
The width and height as defined by the element itself, not imposed by the
surroundings. In CSS2 it is assumed that all replaced elements -- and only
replaced elements -- come with intrinsic dimensions.
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; not all
elements have 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.
Rendered content
The content of an element after the rendering that applies to it according to
the relevant style sheets has been applied. The rendered content of a
replaced element [p. 30] comes from outside the source document.
Rendered content may also be alternate text for an element (e.g., the value
of the HTML "alt" attribute), and may include items inserted implicitly or
explicitly by the style sheet, such as bullets, numbering, etc.
Document tree
The tree of elements encoded in the source document. Each element in this
tree has exactly one parent, with the exception of the root element, which
has none.
Child
An element A is called the child of element B if an only if B is the parent of A.
Descendant
An element A is called a descendant of an element B, if either (1) A is a child
of B, or (2) A is the child of some element C that is a descendant of B.
Ancestor
An element A is called an ancestor of an element B, if and only if B is a
descendant of A.
Sibling
An element A is called a sibling of an element B, if and only if B and A share
the same parent element. Element A is a preceding sibling if it comes before
B in the document tree. Element B is a following sibling if it comes after B in
the document tree.
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