Exclusivobeta
Announcement Bar <mv-announcement-bar>
Barra fina no topo do site para lançamentos, promoções, avisos de manutenção e prévias de recursos, com um botão Dismiss lembrado por id-key em localStorage (um novo id de campanha volta a exibi-la), uma data until opcional que a retira sozinha, tons info, accent e warning, e espaço para uma contagem regressiva em um slot. Os detalhes que os outros erram: ela nunca desloca o layout (a tag recebe estilo e o espaço do botão é reservado antes de o JavaScript rodar, e um script de uma linha no head impede que uma barra dispensada numa visita anterior chegue a ser pintada), é uma região rotulada, e não uma live region, então os leitores de tela não são interrompidos a cada carregamento de página, dispensá-la passa o foco para o próximo elemento focável (ou para <main>) em vez de deixá-lo cair em <body>, e barras consecutivas se empilham como uma única faixa.
| Categoria | Feedback |
|---|---|
| Tipo | Web Component (<mv-announcement-bar>) |
| Status | 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
Instalação
node scripts/add.mjs announcement-bar --out ./src/marvelousAgente de IA com o servidor MCP do Marvelous UI: install_components({ slugs: ["announcement-bar"], target_dir: "<absolute path>/src/marvelous", framework: "react" }).
Arquivos copiados (dependências incluídas): 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
Início rápido, a menor marcação 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>Marcação de referência, para usar como ponto de partida e personalizar com atributos, data-* e variáveis 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.