エクスクルーシブ
Announcement Bar <mv-announcement-bar>
ローンチ、セール、メンテナンスのお知らせ、新機能の予告のためにサイト上部に置く細いバー。Dismiss ボタンの状態は id-key ごとに localStorage に記憶され（新しいキャンペーン id では再び表示されます）、任意の until 日付で自動的に終了し、info、accent、warning のトーンと、スロットに入れるカウントダウンの場所を備えます。
仕組み
ローンチ、セール、メンテナンスのお知らせ、新機能の予告のためにサイト上部に置く細いバー。Dismiss ボタンの状態は id-key ごとに localStorage に記憶され（新しいキャンペーン id では再び表示されます）、任意の until 日付で自動的に終了し、info、accent、warning のトーンと、スロットに入れるカウントダウンの場所を備えます。他の実装が誤りがちな細部：レイアウトを決してずらしません（JavaScript の実行前にタグがスタイル付けされてボタンの場所が確保され、head に置く 1 行のスクリプトにより、以前の訪問で閉じたバーは一度も描画されません）。ライブリージョンではなくラベル付きのリージョンなので、ページを読み込むたびにスクリーンリーダーを遮りません。閉じるとフォーカスを <body> に落とさず、次のフォーカス可能な要素（なければ <main>）に渡します。連続するバーは 1 本の帯として積み重なります。
| カテゴリー | フィードバック |
|---|---|
| タイプ | Web Component（<mv-announcement-bar>） |
| ステータス | 安定版 |
| 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
インストール
node scripts/add.mjs announcement-bar --out ./src/marvelousMarvelous UI の MCP サーバーを使う AI エージェント：install_components({ slugs: ["announcement-bar"], target_dir: "<absolute path>/src/marvelous", framework: "react" })。
コピーされるファイル（依存関係を含む）：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。
使い方
クイックスタート、動作する最小限のマークアップ：
<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>基本となるマークアップです。ここから属性、data-*、CSS 変数でカスタマイズしてください：
<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>Tidewell 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 | タイプ | 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 | タイプ | 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. |
dismissKey | 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. Known limits: when localStorage is blocked (some private modes, strict storage policies), a dismissal lasts only for the current page view; the optional head snippet has to be pasted by hand in the page <head>, the component cannot inject it early enough itself.