What is "tag soup"

n. (context computing Internet informal English) Poorly structured code in a markup language that uses tags (such as HTML), especially when it violates specifications.

In Web development, "tag soup" refers to syntactically or structurally incorrect HTML written for a web page. Because web browsers have historically treated HTML syntax or structural errors leniently, there has been little pressure for web developers to follow published standards, and therefore there is a need for all browser implementations to provide mechanisms to cope with the appearance of "tag soup", accepting and correcting for invalid syntax and structure where possible.

An HTML parser (part of a web browser) that is capable of interpreting HTML-like markup even if it contains invalid syntax or structure may be called a tag soup parser. All major web browsers currently have a tag soup parser for interpreting malformed HTML.

"Tag soup" encompasses many common authoring mistakes, such as malformed HTML tags, improperly nested HTML elements, and unescaped character entities (especially ampersands (&) and less-than signs (<)).

The Markup Validation Service is a resource for web page authors to avoid creating tag soup.