sass-site/source/documentation/values.html.md

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Values

Sass supports a number of value types, most of which come straight from CSS. Every expression produces a value, variables hold values. Most value types come straight from CSS:

• Numbers, which may or may not have units, like 12 or 100px.
• Strings, which may or may not have quotes, like "Helvetica Neue" or bold.
• Colors, which can be referred to by their hex representation or by name, like #c6538c or blue, or returned from functions, like rgb(107, 113, 127) or hsl(210, 100%, 20%).
• Lists of values, which may be separated by spaces or commas and which may be enclosed in square brackets or no brackets at all, like 1.5em 1em 0 2em, Helvetica, Arial, sans-serif, or [col1-start].

A few more are specific to Sass:

• The boolean values true and false.
• The singleton null value.
• Maps that associate values with keys, like ("background": red, "foreground": pink).
• Function references returned by get-function() and called with call().