Exclusivobeta

Announcement Bar `<mv-announcement-bar>`

Barra fina en la parte superior del sitio para lanzamientos, ofertas, avisos de mantenimiento y adelantos de funciones, con un botón Dismiss recordado por id-key en localStorage (un nuevo id de campaña vuelve a mostrarla), una fecha until opcional que la retira sola, tonos info, accent y warning, y espacio para una cuenta regresiva en un slot. Los detalles en los que otros fallan: nunca desplaza el layout (la etiqueta recibe su estilo y el espacio de su botón se reserva antes de que se ejecute JavaScript, y un script de una línea en el head impide que llegue a pintarse una barra descartada en una visita anterior), es una región etiquetada, no una live region, así que los lectores de pantalla no se interrumpen en cada carga de página, al descartarla el foco pasa al siguiente elemento enfocable (o a <main>) en lugar de perderse en <body>, y las barras consecutivas se apilan como una sola banda.

Categoría	Feedback
Tipo	Web Component (`<mv-announcement-bar>`)
Estado	beta
Keywords	exclusive, light, announcement, banner, top-bar, promo, launch, maintenance, dismissible, cls

When to use

A launch, sale or new feature needs a thin site-wide notice above the header that visitors can close for good
A maintenance window or policy change must stay visible until a set date and then disappear without a deploy
A promo should count down to its end inside the top bar and stop showing once the campaign is over
Several notices (a launch and a maintenance warning) may need to stack at the top of the page

Avoid when

The message is about a section or a form and belongs next to that content, with a title and actions → use Alert instead
The feedback follows a user action and should disappear on its own after a few seconds → use Toast instead
A dismissed banner should leave a small badge the user can reopen later → use Grin instead

Instalación

node scripts/add.mjs announcement-bar --out ./src/marvelous

Agente de IA con el servidor MCP de Marvelous UI: install_components({ slugs: ["announcement-bar"], target_dir: "<absolute path>/src/marvelous", framework: "react" }).

Archivos copiados (dependencias incluidas): tokens/tokens.css, core/base.css, core/element.js, core/focus.js, core/motion.js, components/announcement-bar/announcement-bar.js, components/announcement-bar/announcement-bar.css.

Uso

Inicio rápido, el marcado mínimo que funciona:

<mv-announcement-bar id-key="launch-2026-10">Marvelous UI launches Oct 7 · <a href="/waitlist">Join the waitlist</a></mv-announcement-bar>
<!-- optional, in <head>, so a bar dismissed earlier never paints: -->
<script>try{var k=JSON.parse(localStorage.getItem("mv-announcement-bar")||"[]"),s=document.createElement("style");s.textContent=k.map(function(i){return 'mv-announcement-bar[id-key="'+CSS.escape(i)+'"]:not([data-state])'}).join()+"{display:none}";k.length&&document.head.append(s)}catch(e){}</script>

Marcado de referencia: parte de él y personalízalo con atributos, data-* y variables CSS:

<div id="mv-abar-demo">
  <style>
    #mv-abar-demo { display: grid; gap: 1.25rem; width: min(100%, 52rem); margin-inline: auto; }
    #mv-abar-demo .page { overflow: hidden; border: 1px solid var(--mv-border); border-radius: var(--mv-radius-xl); background: var(--mv-bg); box-shadow: var(--mv-shadow-md); }
    #mv-abar-demo .site { display: flex; align-items: center; gap: 1rem; padding: .75rem 1.25rem; border-bottom: 1px solid var(--mv-border); }
    #mv-abar-demo .brand { display: flex; align-items: center; gap: .5rem; font-weight: 650; letter-spacing: -.01em; color: var(--mv-fg); text-decoration: none; border-radius: var(--mv-radius-sm); }
    #mv-abar-demo .brand span { width: 1.25rem; height: 1.25rem; border-radius: .35rem; background: linear-gradient(135deg, var(--mv-accent), var(--mv-accent-fg)); }
    #mv-abar-demo .site nav { display: flex; gap: .15rem; margin-inline-start: auto; }
    #mv-abar-demo .site nav a { padding: .35rem .6rem; border-radius: var(--mv-radius-md); font-size: .875rem; color: var(--mv-fg-muted); text-decoration: none; }
    #mv-abar-demo .site a:focus-visible { outline: 2px solid var(--mv-ring); outline-offset: 1px; }
    #mv-abar-demo main { padding: 1.75rem 1.5rem 2rem; }
    #mv-abar-demo main:focus-visible { outline: 2px solid var(--mv-ring); outline-offset: -2px; }
    #mv-abar-demo h2 { margin: 0 0 .4rem; font-size: 1.4rem; letter-spacing: -.02em; }
    #mv-abar-demo main p { margin: 0; max-width: 34rem; color: var(--mv-fg-muted); font-size: .92rem; line-height: 1.6; }
    #mv-abar-demo .variants { display: grid; gap: .75rem; }
    #mv-abar-demo .variants mv-announcement-bar { border-radius: var(--mv-radius-lg); }
    #mv-abar-demo .foot { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: .75rem; font-size: .8rem; color: var(--mv-fg-subtle); }
    @media (max-width: 36rem) { #mv-abar-demo .site nav a:not(:last-child) { display: none; } }
  </style>

  <div class="page">
    <mv-announcement-bar id-key="launch-2026-10" tone="accent">
      Marvelous UI launches Oct 7 · <a href="#">Join the waitlist</a>
    </mv-announcement-bar>
    <mv-announcement-bar id-key="maintenance-2026-10-04" tone="warning" permanent id="mv-abar-demo-maint">
      Scheduled maintenance Sunday, Oct 4, 2:00-3:00 AM UTC. Checkout may be briefly unavailable.
    </mv-announcement-bar>
    <header class="site">
      <a class="brand" href="#"><span></span>Lumen Studio</a>
      <nav aria-label="Main">
        <a href="#">Pricing</a>
        <a href="#">Docs</a>
        <a href="#">Sign in</a>
      </nav>
    </header>
    <main>
      <h2>Ship your storefront in 12 languages</h2>
      <p>Two bars stacked above the header: the launch notice can be dismissed and stays gone on reload; the maintenance notice has no close button and retires itself when its date passes.</p>
    </main>
  </div>

  <div class="variants">
    <mv-announcement-bar id-key="summer-sale-2026">
      Summer sale: 30% off annual plans · ends in <mv-countdown data-variant="compact" data-offset="187200" to="2026-10-01T18:00:00-04:00" label="Summer sale">Oct 1, 6:00 PM EDT<template slot="ended">a moment</template></mv-countdown> · <a href="#">Upgrade for $84/year</a>
    </mv-announcement-bar>
    <mv-announcement-bar id-key="dark-dashboards" tone="info">
      New: dark mode for dashboards, shared by Aiko Tanaka’s team · <a href="#">See what’s new</a>
    </mv-announcement-bar>
  </div>

  <div class="foot">
    <span>Dismissals are remembered per <code>id-key</code>; a new campaign id shows again.</span>
    <button class="mv-button" data-variant="ghost" data-size="sm" id="mv-abar-demo-reset">Show dismissed bars again</button>
  </div>
</div>

<script type="module">
  // Demo only: keep the dates ahead of today so every bar and timer stays live.
  const root = document.getElementById("mv-abar-demo");
  root.querySelectorAll("mv-countdown[data-offset]").forEach((el) => { el.to = new Date(Date.now() + Number(el.dataset.offset) * 1000).toISOString(); });
  document.getElementById("mv-abar-demo-maint").until = new Date(Date.now() + 3 * 86400000).toISOString();
  document.getElementById("mv-abar-demo-reset").addEventListener("click", () => {
    root.querySelectorAll("mv-announcement-bar").forEach((bar) => bar.reset());
  });
</script>

API

Attributes

Name	Tipo	Default	Description
`id-key`	string		Campaign id the dismissal is remembered under (localStorage key mv-announcement-bar, a JSON list of the last 50 dismissed ids). Change it for a new campaign and the bar shows again. Without it, a hash of the bar’s own text is used (the head script then cannot hide it before paint).
`until`	ISO 8601 date-time		Moment after which the bar hides itself, on load or live while the page is open (e.g. 2026-10-14T23:59:00+02:00). Invalid or missing: no end.
`tone`	info \| accent \| warning	`(neutral ink)`	Color: neutral dark ink, accent, tinted info or tinted warning. Works in light and dark themes.
`permanent`	boolean		No Dismiss button and stored dismissals are ignored (a maintenance notice that must stay until `until`).
`data-state`	open \| dismissed		Set by the component on upgrade (for styling); the head script only hides bars that do not have it yet.

Properties

Name	Tipo	Description
`idKey / until / permanent`	string / string / boolean	Mirror the attributes.
`strings`	object	Override default texts: label (region name, “Announcement”) and dismiss (button name, “Dismiss announcement”). An aria-label or aria-labelledby you set on the bar is kept.
`key`	string	Key the dismissal is stored under: id-key, else auto-`<hash of the text>` (read-only).
`dismissed`	boolean	True while the bar is hidden because it was dismissed, remembered or expired (read-only).
`MvAnnouncementBar.earlyScript`	string (static)	The `<head>` snippet as a string, to inline from a server template or framework layout. Also exported as EARLY_SCRIPT.

Methods

Name	Description
`dismiss()`	Hides the bar (height collapse), remembers its key and moves focus on if it was inside; returns false if vetoed or already hidden.
`reset()`	Forgets the dismissal for this key and shows the bar again (unless until has passed).

Events

Name	Description
`mv-dismiss`	detail = { key, reason: "user" \| "expired" }. Cancelable when reason is user: preventDefault() keeps the bar open and nothing is stored. Also fired (not cancelable) when until passes while the bar is visible.

Content structure

Name	Description
`(content)`	The message: text, links, a `<mv-countdown data-variant="compact">` or any inline content. Nested custom elements are ignored when hashing the text for the automatic key.

CSS classes

Name	Description
`mv-announcement-bar-dismiss`	Generated Dismiss button (end side, aligned to the first line).

CSS variables

Name	Default	Description
`--mv-announcement-bar-bg`	`per tone`	Background color.
`--mv-announcement-bar-fg`	`per tone`	Text, link and icon color.
`--mv-announcement-bar-border`	`per tone`	Bottom hairline color (transparent for the neutral and accent tones).
`--mv-announcement-bar-height`	`2.5rem`	Minimum height, reserved before JavaScript runs.

Accessibility

The bar gets role=region with the accessible name “Announcement” (strings.label, or your own aria-label), so it is listed with landmarks and can be skipped, but it is not a live region: nothing is read out on page load or navigation. Put it first in <body>, before the header, so it comes first in reading and tab order. The Dismiss button is a real <button> named “Dismiss announcement” (strings.dismiss), the × icon is aria-hidden, the glyph stays small while the hit area grows to about 42 px, focus is a visible 2 px outline in the bar’s text color on every tone. When the bar is dismissed (or expires) while focus is inside it, focus moves to the next focusable element after it (the next stacked bar, the logo, the nav), else to <main> (made focusable with tabindex=-1 for that moment), never to <body>; the bar is inert while it collapses. The collapse is a short height and opacity animation, instant under prefers-reduced-motion or data-motion=reduce. No layout shift: the tag is styled and the button space reserved by CSS alone, so the upgrade changes nothing; for returning visitors, the optional <head> snippet (quickstart, or MvAnnouncementBar.earlyScript) hides dismissed id-keys before first paint. Forced colors: system border and button colors. Hidden when printing. Dismissals sync across open tabs. SSR-safe: nothing touches window or document at import, and the content is plain HTML readable without JavaScript.