Control characters are non-printable characters that are typically used for communication and device control, as format effectors, and as information separators.

In SGML applications, the use of control characters is limited in order to maximise the chance of successful interchange over heterogenous networks and operating systems. In HTML, only three control characters are used: Horizontal Tab (HT, encoded as 9 decimal in US-ASCII and ISO-8859-1), Carriage Return, and Line Feed.

Horizontal Tab is interpreted as a word space in all contexts except preformatted text. Within preformatted text, the tab should be interpreted to shift the horizontal column position to the next position which is a multiple of 8 on the same line; that is, col := ((col+8) div8) * 8 (where div is integer division).

Carriage Return and Line Feed are conventionally used to represent end of line. For Internet Media Types defined as "text/*", the sequence CR/LF is used to represent an end of line. In practice, text/html documents are frequently represented and transmitted using an end of line convention that depends on the conventions of the source of the document; frequently, that representation consists of CR only, LF only, or CR/LF combination. In HTML, end of line in any of its variations is interpreted as a word space in all contexts except preformatted text. Within preformatted text, HTML interpreting agents should expect to treat any of the three common representations of end-of-line as starting a new line.

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