An anchor or placemark link in an HTML document is a named location (bookmark) that can be referenced by a URL. The location is named by the id attribute of the <a/> tag and referenced by the hash ("#") fragment identifier in a URL. Most browsers will automatically scroll a web page to the indicated location on the page.
An attribute is a named property associated with an element. In a marked-up document, attributes are coded between the element name and the tag's terminating /> or > in a start tag and is represented by the attribute name, an equal sign (=) and the value of the property enclosed in quotes.
A boolean attribute is a property associated with an element that represents either a true or false value. In a polyglot HTML document, a boolean attribute with a true value is coded with a value that matches the attribute name, as in attrname="attrname". A boolean attribute with a false value should be completely omitted.
Child elements are the elements that are in the content of a parent element. In an HTML document, the child elements are enclosed between the parent element's start tag and it's end tag.
The Document Object Model is a well-defined model for SGML and XML documents, which can be "marked up" with tags such as in HTML. While actual documents are often viewed by authors as text into which the markup tags have been inserted, the DOM focuses more on the tags themselves, which are represented in the model by elements, and their content.
An element is an object in the hierarchical model of a document. It always has an element name and may also have attributes and/or child elements. The difference between an element and a tag is that an "element" is a more abstract representation of a node in the hierarchical structure of a document that encompasses its attributes, child elements and other content. In an actual document, an element is explicitly represented by either a start tag and an end tag enclosing its child elements and other content, if any, or simply by an empty tag if it has no content.
In some cases, an element is implied even when the corresponding tag does not appear in the document. A good example of this is the <tbody> tag, which is often an implied child element between a <table> tag and a <tr> table row tag.
A node is the representation of an object in the hierarchical model of a document. Types of nodes in a DOM include:
These nodes provide a model for content between the ">" ending one tag in a marked-up document and the "<" beginning another tag, which includes whitespace, text, character references and character data sections.
A polyglot HTML document is an HTML document that has been coded in such a way that it can be successfully parsed as either a text/html document or as an XML type such as application/xhtml+xml. This requires following the subset of rules that are common to both serializations of HTML 5 and avoiding anything that is unique to one syntax or the other.
A significant advantage of writing documents using polyglot HTML syntax is that, by definition it's a common subset of the other possible syntaxes, and so there are fewer rules for newcomers beginning to code HTML to learn. Then if at some point later you find you need a feature of one of the more complete HTML syntax alternatives for some task you are trying to accomplish, you'll be able to determine which of the two possible HTML syntax options will suit your needs at the time.
The root node of the Document Object Model is a single node that is the parent of the nodes at the top of a document's nested tag hierarchy, the nodes at the beginning of the document, which includes a single top element. In an HTML document the root node is the parent of the element represented by the <html> tag and of any other top-level nodes such as processing instructions.
A tag in HTML is a representation of an element in a document. A start tag or empty tag contains a <, the tag name, one or more optional attribute/value pairs separated from the tag name and other attributes by whitespace and ends with either > or />. An end tag contains just </, the tag name and >.
The top element is a single node in the DOM that represents a tag that spans from the beginning of the document to the end of it. The parent node of the top element is the root node. In an HTML document, the top element represents the <html> start tag at the beginning of a document and the </html> end tag at the end of the document, which enclose all other tags in the document.
A void element is an element that can never have any child elements. A void element should always be coded as an empty tag (<br/>, <img .../>) rather than with a start tag and an end tag (<br></br>, <img ...></img>).