feat: add template-no-autoplay

[johanrd]

Note

This is part of a series where Claude has audited eslint-plugin-ember against jsx-a11y, vuejs-accessibility, angular-eslint, lit-a11y and html-validate, ember-template-lint, and the HTML and WCAG specs.

See html-validate no-autoplay for the peer rule concept.

Adds template-no-autoplay: flags autoplay on <audio> / <video> when the attribute is statically present.

Premises

Premise 1 (HTML): The HTML spec's autoplay attribute is a boolean attribute: presence alone means "true", regardless of value. <audio autoplay="false"> reads .autoplay === true at the DOM level (verified in Chrome).

Premise 2 (WCAG): WCAG 2.1 SC 1.4.2 Audio Control (Level A) applies to audio that plays automatically for more than 3 seconds. The W3C ACT rule aaa1bf that operationalizes SC 1.4.2 is explicitly inapplicable when muted is true or the media has no audio track. <video autoplay muted> (GIF-style) is out of SC 1.4.2's scope. Silent-motion concerns fall under SC 2.2.2 Pause/Stop/Hide, which depends on runtime facts static analysis cannot determine.

Conclusion: Flag <audio> with a statically-present autoplay. Flag <video> with a statically-present autoplay unless a statically-truthy muted is also present. Skip mustache-unknown values — false positives are worse than false negatives. Recognize explicit {{false}} / {{"false"}} literals as opt-outs.

Flags

<audio autoplay></audio>
<video autoplay src='/intro.mp4'></video>
<audio autoplay='false'></audio>              {{! boolean attr — any presence is truthy }}
<video autoplay={{true}}></video>
<video autoplay muted={{false}}></video>
<audio autoplay muted></audio>                {{! muted exception is video-only }}

Allows

<audio src='/track.mp3' controls></audio>
<video autoplay muted></video>                {{! GIF-style: muted video is out of SC 1.4.2 scope }}
<video autoplay muted loop playsinline></video>
<audio autoplay={{this.shouldAutoplay}}></audio>   {{! unknown mustache — skipped }}
<video autoplay={{false}}></video>            {{! explicit opt-out literal }}
<audio autoplay={{"false"}}></audio>

Schema option

additionalElements: string[] extends the default {audio, video} set. The muted exception applies only to <video>; elements added via additionalElements are flagged on autoplay alone (their semantics are unknown).

Prior art

Plugin	Equivalent	Verified behavior
jsx-a11y	—	No equivalent rule (media-has-caption covers captions, not autoplay).
vuejs-accessibility	—	No equivalent rule.
lit-a11y	—	No equivalent rule.
@angular-eslint/template	—	No equivalent rule.
html-validate	no-autoplay	Reports autoplay on configured tags; include/exclude options; skips DynamicValue. No muted-video exception.

Notes

• <video autoplay muted> is not flagged — aligned with W3C ACT aaa1bf's scoping of SC 1.4.2 to audio output. <audio autoplay muted> is still flagged — the combination is spec-nonsensical and strongly suggests author confusion.
• SC 2.2.2 (motion pause/stop/hide) is not enforced statically — duration, parallel-content layout, and essentialness are runtime concerns.
• Defaults to {audio, video} rather than html-validate's any-element default, because Ember codebases commonly pass autoplay as a non-media component arg (e.g. <Carousel autoplay={{...}} />).
• Opt-in: not added to any preset config.