The less namespace

The rules of the core under @stylistic/less/<rule>, for stylesheets written in Less and parsed with postcss-less. The recommended preset sets them under these names over **/*.less and names no package, so a stylesheet is read by these rules as soon as the project tells Stylelint to parse its Less files as Less — which is what Your own syntaxes shows. A project that names no syntax has its .less files read as CSS by the @stylistic/ names, and nothing warns it about that.

Every rule of the core is here, with the same options, upper of at-rule-name-case aside, and the same documentation, and plain CSS is read exactly as the core reads it — a plain block of an HTML page beside a lang="less" one included, so a mixed page is configured with these rules alone, but for upper of at-rule-name-case, which its plain blocks take from the core's copy. What the namespace reads beyond the core:

• the constructs postcss-less hands over — mixins and their calls, variables (@foo: bar;), detached rulesets, :extend, CSS guards (when), maps — are passed over the way the core passes over what is not standard CSS, rather than misread as code;
• an inline comment (//) is a comment of the text a rule reads, and every fix keeps out of one;
• --fix keeps the whitespace around a mixin call's !important on the side of the flag the file puts it, except that an end-of-line comment behind the flag still gets the flag moved behind it;
• declaration-block-trailing-semicolon under never reports and leaves the semicolons Less will not part with: behind an at-rule without a block (a { @extend .b } is an error to Less), a variable (@v: pink), and a declaration with no value (color:;), where a custom property's lone !important counts as a value; the semicolon behind a mixin call or a call to a detached ruleset, @name() and @name()[key] alike, goes as it does behind any declaration, while @name () with a space is an at-rule;
• at-rule-name-space-after and at-rule-name-newline-after report an @import or @plugin with no whitespace behind the name, @import(reference) "x.less" or @plugin"p", and write none into it, since Less takes the two as directives only with whitespace there;
• at-rule-name-case takes lower alone, since Less refuses an at-rule name holding an upper-case letter;
• indentation measures the head of a mixin definition as it measures an at-rule's parameters: a line continuing it outside the parameter list, a guard on a line of its own among them, stands a level deeper, and a line inside the list follows the parentheses as a mixin call's arguments do;
• unit-case reads and fixes the value of a variable (@v: 10PX) as it does a declaration's, whatever whitespace stands around the colon;
• unit-case reads a dimension as Less does: the number is digits with at most one period, and the unit ends at the first escape, hyphen or digit, so 10PX-2REM has the units PX and REM and 1E5PX the units E and PX, where the core reads the exponent into the number and the rest as one unit;
• the value-slash-space-* and media-feature-slash-space-* rules read a solidus outside parentheses as the separator it is to the core, 2/@a included, since Less divides only inside parentheses under its default math mode; a parenthesized group is passed over, and a variable's own value (@a: 1/2) is no declaration to them;
• named-grid-areas-alignment under alignColumns leaves a row without a line name at the indentation, since the empty list of names [] that would put it under the others is an error to Less.

This namespace is about the stylistic side of the CSS a Less file holds. The Less constructs themselves — variables, mixins, guards — have no stylistic rules here.