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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 ...
On the opposite, the code point U+0085 is a valid control character in Unicode and ISO/IEC 10646, as well as in XML 1.0 and XML 1.1 documents (in all contexts), and its usage is not discouraged (it is treated as whitespace in many XML contexts, or as a line-break control similar to U+000D and U+000A in preformatted texts in some XML applications).
HTML and XML provide ways to reference Unicode characters when the characters themselves either cannot or should not be used. A numeric character reference refers to a character by its Universal Character Set/Unicode code point, and a character entity reference refers to a character by a predefined name. A numeric character reference uses the ...
The SGML-based markup languages allow document authors to use special sequences of characters from the ASCII range (the first 128 code points of Unicode) to represent, or reference, any Unicode character, regardless of whether the character being represented is directly available in the document's encoding. These special sequences are character ...
In character data and attribute values, XML 1.1 allows the use of more control characters than XML 1.0, but, for "robustness", most of the control characters introduced in XML 1.1 must be expressed as numeric character references (and #x7F through #x9F, which had been allowed in XML 1.0, are in XML 1.1 even required to be expressed as numeric ...
But in practice, and required for HTML and XML documents, ISO-8859-6 also stands for logical order text. There is also ISO-8859-6-E which supposedly requires directionality to be explicitly specified with special control characters; this latter variant is in practice unused.
But usually in practice, and required for XML documents, [citation needed] ISO-8859-8 also stands for logical order text. The WHATWG Encoding Standard used by HTML5 treats ISO-8859-8 and ISO-8859-8- I as distinct encodings with the same mapping due to influence on the layout direction, but notes that this no longer applies to ISO-8859-6 (Arabic ...
An "encoding sniffing algorithm" is defined in the specification to determine the character encoding of the document based on multiple sources of input, including: Explicit user instruction An explicit meta tag within the first 1024 bytes of the document