ADR 145: the unnamed tag and defaultTag

Status: accepted 2026-10-04 (decision 145 and its two addenda in the decisions log). Followed by: ADR 146 (:name), ADR 147 (wildcard children).

Context

Marko’s parser writes div into the AST for a tag that has only a shorthand: <#main>, <.card>, concise #main. The literal lives in @marko/compiler (tagName.value ||= "div" in the parser) because Marko has one host and div is HTML’s generic container. MX has many targets, including data, where div means nothing.

Seen as a language, the construct is “an unnamed tag carrying an id or class shorthand”, and div is HTML’s answer, not the language’s. Someone has to resolve the name; the question was who.

Decision

The unnamed tag is resolved through defaultTag, in this order:

1. the parent’s contract: defaultTag declared alongside children (sidecar or mx.contracts), honoured only when the target’s declarations permit per-tag override (a flag on its declarations; data permits, HTML-emitting hosts default to permit);
2. the package’s user override, package.json#mx.<target>.defaultTag;
3. the host’s optional override on its descriptor;
4. the target’s built-in, required on every target descriptor: div for the html target (so every HTML-emitting host and every existing template is byte-identical), object for the data target.

object is a built-in tag of the data target: the anonymous node, carrying the shorthand’s id/class, open contract, always known. It is never an unknown-tag error and needs no declaration. A closed parent children that lists neither object nor a defaultTag yields the ordinary E2 error.

#x still becomes id="x" and .a.b still becomes class="a b", as in Marko. After resolution the tag is ordinary: the parent’s closed children, its own attributes contract (<.x> under a tag whose closed attributes lack class is an E1 error), attribute tags and everything else apply unchanged.

From the user’s point of view there is one error, at the declaration, never at the use site: an invalid defaultTag value, meaning a name that is not an element of the target or a custom tag reachable from that package (on the html family, an element Marko’s html, svg or math taglibs define; on the JSX hosts, Solid and Angular also a dashed custom-element name such as sl-card, because an unknown dashed name compiles to a native element there; on data, object or a custom tag), or a tag whose parse shape on that target is not plain (for html: void, text, statement, control-flow or whitespace-preserving tags such as input, title, script, textarea, pre, and Marko core tags such as await).

Why post-parse

Marko has no hook that fires before div is written: taglib migrate, transform and parseOptions all run after onOpenTagName. A pre-parse source rewrite was rejected: it shifts every span and cannot see the parent, which the resolution needs. The name node Marko leaves has an empty source span, which no named tag has; that is the reliable mark. Core detects it in lowering, where the parent chain is known, and asks a generic resolver; the ladder above lives in the registry and the targets, so core stays host-agnostic (no div literal in @mxlang/core). MX never runs Marko’s translator, so the parser’s div leaks nowhere; its only parse-time use is the parse options of div, which is why a defaultTag must be a plain-parsing tag.

Alternatives considered

Consequences

• Divergence row: Marko always resolves the unnamed tag to div; MX resolves it by vocabulary.
• Target descriptors gain a required defaultTag; host descriptors an optional override, and the target’s declarations a permit flag; contracts a defaultTag key next to children; the registry refuses a target descriptor without one, third-party targets included.
• With ADR 146, <:title type="string"/> under attributes that declares defaultTag: "attribute" is <attribute name="title" type="string"/>.