Extend and :is() wrapping

Jess writes extend with the $extend form. $extend defaults to partial (compound-subset) matching — the equivalent of Less's :extend(... all):

.a > .c {
  color: red;
}
.x {
  $extend .c;         // partial (default): matches the .c compound anywhere
}

.a > :is(.c, .x) {
  color: red;
}

When the target matches only part of a compound selector, Jess grafts :is(<matched>, <extender>) into that position, preserving the surrounding context (.a > stays on the left). When the target matches a whole compound, the extender is appended to the selector list instead (no :is() wrapper).

Grouping the match and the extender into :is() also raises specificity: an :is() scores as its most specific argument, so .a > :is(.c, #b) scores an ID even for the plain-class .c branch. See Selector compaction — Specificity and :is() grouping for the cascade implications and the vs-4.x migration note.

Exact matching

Use !exact to require a whole-selector match — the equivalent of Less's plain :extend(...) without all:

.x {
  $extend .c !exact;   // only matches a bare `.c` compound, appends to the list
}

Notes

• Matching runs on compiled selectors (after nesting/parent resolution), never on source text.
• An interpolated selector as a match target matches nothing; an interpolated extender works.
• Extend is selector-level — it shares a rule's whole declaration block and cannot pull in a single property.
• Target matching is exact apart from attribute-quote normalization: a leading star, pseudo-class order, and nth form all matter.
• An extend targeting a (reference)-imported rule surfaces the declarations under your selector only; the referenced target header is never emitted.

This is the same extend engine documented for Less 5.x. For the full behavior — :is() grafting, sibling compaction, nested re-nesting and its flatten triggers, @media scoping, and reference visibility — see the Less Extend and :is() Wrapping and Extend Semantics pages.