In web development, "tag soup" is a pejorative for syntactically or structurally incorrect HTML written for a web page. Because web browsers have historically treated structural or syntax errors in HTML 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, with most error-handling elements standardized.
"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 (<)).
I have used this term in my instruction for years to characterize the jumble of angle brackets acting like tags in HTML in pages that are accepted by browsers. Improper minimization, overlapping constructs ... stuff that looks like SGML markup but the creator didn't know or respect SGML rules for the HTML vocabulary. In effect a soupy collection of text and markup. [...] I've never seen the term defined anywhere.
The Markup Validation Service is a resource for web page authors to avoid creating tag soup.
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