Selectors

Scraping KDL accepts a subset of CSS. The subset operates in the same manner in the internal DOM and in a live browser. The compiler rejects a selector outside of the subset. Thus you do not find the difference at execution time on one runtime only.

The full profile is in selectors-v0.1.md.

What you can use

The profile contains the parts of CSS that each engine implements in the same manner:

• the universal selector *, a type selector such as div, an ID selector such as #main, and a class selector such as .entry;
• the attribute selectors: the presence form [href] and the operators =, ~=, |=, ^=, $=, and *=;
• the combinators: descendant, child >, adjacent sibling +, and general sibling ~;
• a selector list with commas;
• the structural pseudo-classes: :first-child, :last-child, :only-child, :empty, :first-of-type, :last-of-type, :only-of-type, :nth-child(An+B), :nth-last-child(An+B), :nth-of-type(An+B), :nth-last-of-type(An+B), and :not(compound-selector).

select "table.entries tbody tr:not(.header)"

What the compiler rejects

The compiler rejects these constructions with the diagnostic E_SELECTOR_UNSUPPORTED:

• each pseudo-element;
• :has(), :is(), :where(), and :scope;
• a shadow DOM selector and a vendor pseudo-selector;
• a namespace selector;
• a CSS escape sequence in an identifier or a string token;
• the case-sensitivity flags i and s in an attribute selector;
• a pseudo-class of the user interface state, such as :hover, :focus, :visited, and :checked.

A selector with bad syntax gets the different diagnostic E_SELECTOR_INVALID.

The rejection occurs at compile time, with the position of the character in the selector:

extractor.kdl:9:5: error E_SELECTOR_UNSUPPORTED: selector byte 9: unsupported pseudo-class "has" [output.title.selection]

An implementation can use a larger selector engine internally. It must still reject each selector outside of the profile under the language version 2026-07-15. Thus a program that compiles on one runtime also compiles on the other runtime.

Why the profile is small

A pseudo-class of the user interface state, such as :hover, has a value only in a live browser. A structural pseudo-class gives the same result in each engine.

If the two classes were in one profile, a program could operate correctly in browser mode and fail in HTTP mode. The profile prevents this condition. Each accepted selector has the same meaning in the two modes.

Alternatives to :has()

The pseudo-class :has() is the most frequent absent function. Use one of these methods:

• Select the parent element with a collection, then use a field in the row to find the child. A row without the child gives a missing value, and you control that condition with required.
• Select the child element directly when you need only its value. The parent element is frequently not necessary.
• In browser mode only, use evaluate-js with scope="current". This is an intentional escape from the portable profile. It needs the capability browser.evaluate-js and an explicit opt-in for JavaScript.

Cardinality

The property match of the node select controls the number of the elements:

Value	Behavior
one (default)	Exactly one element. Zero elements is a missing value. Two or more elements causes E_SELECTOR_CARDINALITY.
first	The first element in document order. Zero elements is a missing value.

A collection is different. Its select gives each element that agrees with the selector, in document order. One row comes from each element.

Use match="one" when the page must have exactly one element. The runtime then tells you when the structure of the page changed. Use match="first" only when more than one element is correct and you want the first element.

Semantics

• The matches follow the tree order of the DOM.
• A selector list gives the elements in document order and has no duplicates.
• The names of the elements and the attributes follow the ASCII case-insensitive rules of HTML.

Next step

• Transforms — how to make a value from the selected text.
• Browser Mode — the selector behavior with a live page.