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Unnecessary use of HTML character references may significantly reduce HTML readability. If the character encoding for a web page is chosen appropriately, then HTML character references are usually only required for markup delimiting characters as mentioned above, and for a few special characters (or none at all if a native Unicode encoding like ...
However, all valid characters and sequences in the UCS, including all bidirectional controls or private-use assignments (but with the exception of non-whitespace C0 and C1 controls, non-characters, and surrogates) are also usable and valid in HTML, XML, XHTML and MathML, either in plain-text values of attributes or in text elements (by encoding ...
In SGML, XML, and HTML, the ampersand is used to introduce an SGML entity, such as (for non-breaking space) or α (for the Greek letter α). The HTML and XML encoding for the ampersand character is the entity &. [38] This can create a problem known as delimiter collision when converting text into one of these markup languages.
Web pages authored using HyperText Markup Language may contain multilingual text represented with the Unicode universal character set.Key to the relationship between Unicode and HTML is the relationship between the "document character set", which defines the set of characters that may be present in an HTML document and assigns numbers to them, and the "external character encoding", or "charset ...
There is another kind of character reference called a character entity reference, which allows a character to be referred to by a name instead of a number. (Naming a character creates a character entity.) HTML defines some character entities, but not many; all other characters can only be included by direct encoding or using NCRs.
URL encoding, officially known as percent-encoding, is a method to encode arbitrary data in a uniform resource identifier (URI) using only the US-ASCII characters legal within a URI. Although it is known as URL encoding , it is also used more generally within the main Uniform Resource Identifier (URI) set, which includes both Uniform Resource ...
The quoted-printable encoding uses the equals sign as an escape character. URL and URI use % - escapes to quote characters with a special meaning, as for non-ASCII characters. The ampersand ( & ) character may be considered as an escape character in SGML and derived formats such as HTML and XML .
In contrast, a character entity reference refers to a character by the name of an entity which has the desired character as its replacement text. The entity must either be predefined (built into the markup language) or explicitly declared in a Document Type Definition (DTD). The format is the same as for any entity reference: &name;