HTML element

HTML Elements and Structure
– HTML elements are components of HTML documents.
– HTML elements can add semantics and formatting to parts of a document.
– Elements can have attributes specified.
– Elements can have content, including other elements and text.
– HTML documents are composed of a tree of HTML nodes.
– Most HTML elements are written with a start tag and an end tag.
– Some elements, called void elements, do not have an end tag.
– Void elements have predefined behavior and cannot contain content or other elements.
– XHTML requires opening and closing all elements, including void elements.
– An alternative way to specify a void element is to use a self-closing tag.
– The root element of an HTML document is the HTML element.
– The HTML element delimits the beginning and end of an HTML document.
– The head element contains metadata about the document.
– The body element contains the visible content of the document.
– The title element specifies the title of the document.

Elements vs. Tags and SGML vs. XML
– The position of an element is indicated by start and end tags.
– The distinction between elements and tags is emphasized in HTML specifications.
– Certain tags can be omitted, but the element is still present.
– HTML parsing depends on the Document Type Definition (DTD).
– The DTD specifies the valid combinations of elements in a document.
– SGML is complex, while XML is a simpler alternative.
– XML parsing is simpler than SGML parsing.
– HTML on the web is often treated as XML (XHTML) or HTML5.
– Parsing document tags into DOM elements is simplified in XML and HTML5.

CSS and Block Elements
– CSS defines block elements with the ‘display: block;’ declaration.
– HTML has %block; and %inline; groups for block-level and inline elements.
– Block-level elements cannot be placed into an inline context.
– Block and inline elements have different CSS behaviors by default.
– CSS behavior can be changed from the default for specific elements.
– Block elements have a rectangular structure and can span the entire width of their parent element.
– Block elements have content, padding, border, and margin, forming the box model.

HTML Element Standards and Status
– HTML elements are defined in a series of open standards issued since 1995.
– User agents often developed their own elements during the browser wars of the 1990s.
– XML introduced mechanisms to allow anyone to develop their own elements and incorporate them in XHTML documents.
– HTML 4.01 was rewritten in an XML-compatible form called XHTML 1.0.
– XHTML 1.0 documents are valid or nearly valid HTML 4.01 documents.
– Several elements have become deprecated or invalid in later standards.
– HTML 4.01 / XHTML 1.0 introduced three types of DTD: Transitional, Frameset, and Strict.
– HTML5 provides a listing of obsolete features, both conforming and non-conforming.
– The frame elements are highly problematic for user accessibility.
– XHTML 1.1 does not include frames at all.

Additional HTML Elements
– HTML elements can be written in any combination of upper or lower case, but must be in lower case in XHTML.
– There are three kinds of HTML elements: normal elements, raw text elements, and void elements.
– Void elements only have a start tag and may not contain any children.
– Boolean attributes do not require a value to be specified.
– HTML allows replacement content for non-visual user agents and can be extended through scripts.
– The document head elements include base, basefont (deprecated), isindex (deprecated), link, and meta.
– The document body elements include block elements, inline elements, basic text elements, lists, and images.
– The basic text elements include p element for paragraphs and h1-h6 elements for section headings.
– The list elements include dl for description lists, ul for unordered lists, and ol for ordered lists.
– The definition list elements include dt and dd.
– Other block elements include address, article, aside, blockquote, and div.
– Additional block elements include figure, figcaption, footer, header, and hr.