Invalid HTML

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 (<)).

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.

"Tag soup" is a term used to denigrate various practices in web authoring. Some of these (roughly ordered from most severe to least severe) include:

Malformed markup is arguably the most severe problem in web authoring. However, thanks to better education and information and perhaps with some help from XHTML, the issue of malformed markup is becoming less common. Browsers, when faced with malformed markup, must guess the intended meaning of the author. They must infer closing tags where they expect them and then infer opening tags to match other closing-tags. The interpretation can vary markedly from one browser to the next. Ian Hickson wrote a detailed article investigating the differences between how browsers handle tag soup.

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