CSS Anchor Positioning & Popover API: No More JS for Popups in 2026

前端工程

CSS Anchor Positioning: The "Emancipation Proclamation" for Frontend Devs

Every frontend developer has experienced this pain: using JS to calculate Tooltip positions, handling boundary overflow, listening to scroll and resize events... Now, CSS can handle all of this natively.

Historical review: In 2016 we used Popper.js, in 2020 Floating UI, and in 2026 we finally have native CSS anchor positioning. After 10 years, popup positioning is no longer a JS job.

Evolution from JS Positioning to CSS Positioning

2016    jQuery offset() + manual calculation
        Recalculate on every scroll/resize

2020    Floating UI / Popper.js
        Auto-flip, offset, boundary detection
        But still requires JS init and destroy

2024    CSS Anchor Positioning (Chrome 125+)
        Pure CSS declarative positioning
        Browser handles boundaries and scrolling natively

2026    Anchor Positioning + Popover API
        Pure CSS popups: zero-JS toggle, zero-JS positioning
        Complete declarative popup solution

anchor-name and position-anchor

Basic Usage

<div class="card">
  <button class="trigger">More Actions</button>
  <div class="dropdown">Dropdown content</div>
</div>

.trigger {
  anchor-name: --my-anchor;
}

.dropdown {
  position: fixed;
  position-anchor: --my-anchor;
  top: anchor(bottom);
  left: anchor(left);
}

anchor() Function Reference

.tooltip {
  position: fixed;
  position-anchor: --trigger-anchor;

  /* Relative to anchor edges */
  top: anchor(bottom);
  bottom: anchor(top);
  left: anchor(left);
  right: anchor(right);

  /* Anchor center points */
  left: anchor(50%);
  top: anchor(50%);

  /* With offset */
  top: calc(anchor(bottom) + 8px);
  left: calc(anchor(left) - 4px);
}

position-area Nine-Grid Positioning

Nine-Grid Syntax

┌─────────────┬─────────────┬─────────────┐
│  top left   │   top center│  top right  │
│             │             │             │
├─────────────┼─────────────┼─────────────┤
│ center left │   center    │ center right│
│             │             │             │
├─────────────┼─────────────┼─────────────┤
│ bottom left │bottom center│ bottom right│
│             │             │             │
└─────────────┴─────────────┴─────────────┘

.tooltip-below {
  position-area: bottom center;
}

.tooltip-right {
  position-area: center right;
}

.dropdown-below-left {
  position-area: bottom left;
}

position-fallback and Multi-Level Fallback Strategy

Boundary Overflow Auto-Flip

.dropdown {
  position: fixed;
  position-anchor: --trigger;

  /* Preferred: below anchor */
  top: anchor(bottom);
  left: anchor(left);

  position-fallback: --dropdown-fallback;
}

@position-fallback --dropdown-fallback {
  /* Fallback 1: above anchor */
  @try {
    bottom: anchor(top);
    left: anchor(left);
  }

  /* Fallback 2: below anchor, right-aligned */
  @try {
    top: anchor(bottom);
    right: anchor(right);
  }

  /* Fallback 3: above anchor, right-aligned */
  @try {
    bottom: anchor(top);
    right: anchor(right);
  }
}

Popover API: Native Popover Layer

Basic Usage

<button popovertarget="my-popover">Open Popover</button>

<div id="my-popover" popover>
  <p>This is native Popover content</p>
  <button popovertarget="my-popover" popovertargetaction="hide">Close</button>
</div>

[popover] {
  position: fixed;
  inset: unset;
  margin: 0;
  padding: 16px;
  border: 1px solid #e5e7eb;
  border-radius: 8px;
  background: white;
  box-shadow: 0 10px 25px rgba(0, 0, 0, 0.15);
}

Popover + Anchor Combination

<button class="menu-trigger" popovertarget="menu-popover">Menu</button>

<div id="menu-popover" popover class="menu-dropdown">
  <a href="/profile">Profile</a>
  <a href="/settings">Settings</a>
  <a href="/logout">Logout</a>
</div>

.menu-trigger {
  anchor-name: --menu-anchor;
}

.menu-dropdown {
  position-anchor: --menu-anchor;
  position-area: bottom left;
  margin: 4px;
}

anchor-size(): Auto-Size Based on Anchor Element

.trigger {
  anchor-name: --select-anchor;
}

.select-dropdown {
  position: fixed;
  position-anchor: --select-anchor;
  top: anchor(bottom);
  left: anchor(left);

  /* Dropdown width follows trigger button */
  width: anchor-size(width);

  /* Or set minimum width */
  min-width: anchor-size(width);
}

Practice: Pure CSS Common UI Components

Tooltip

<span class="tooltip-trigger" tabindex="0">
  Help
  <span class="tooltip-content" role="tooltip">This is a help tooltip</span>
</span>

.tooltip-trigger {
  anchor-name: --tt-anchor;
  position: relative;
  cursor: help;
  text-decoration: underline dotted;
}

.tooltip-content {
  position: fixed;
  position-anchor: --tt-anchor;
  position-area: top center;
  margin: 8px;
  padding: 6px 10px;
  background: #1f2937;
  color: white;
  border-radius: 4px;
  font-size: 13px;
  white-space: nowrap;
  pointer-events: none;
  opacity: 0;
  transition: opacity 0.15s;

  position-fallback: --tt-fallback;
}

.tooltip-trigger:hover .tooltip-content,
.tooltip-trigger:focus .tooltip-content {
  opacity: 1;
}

@position-fallback --tt-fallback {
  @try { position-area: bottom center; }
  @try { position-area: center right; }
  @try { position-area: center left; }
}

<button class="dropdown-trigger" popovertarget="dd-menu">
  Options ▾
</button>
<div id="dd-menu" popover class="dropdown-menu">
  <button class="dropdown-item">Edit</button>
  <button class="dropdown-item">Copy</button>
  <button class="dropdown-item">Delete</button>
</div>

.dropdown-trigger {
  anchor-name: --dd-anchor;
}

.dropdown-menu {
  position-anchor: --dd-anchor;
  position-area: bottom left;
  margin: 4px;
  width: anchor-size(width);
  min-width: 160px;
  padding: 4px;
  border-radius: 8px;
  background: white;
  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.12);

  position-fallback: --dd-fallback;
}

.dropdown-item {
  display: block;
  width: 100%;
  padding: 8px 12px;
  border: none;
  background: none;
  text-align: left;
  border-radius: 4px;
  cursor: pointer;
}

.dropdown-item:hover {
  background: #f3f4f6;
}

@position-fallback --dd-fallback {
  @try { position-area: top left; }
}

Comparison with Floating UI/Popper.js

Dimension CSS Anchor + Popover Floating UI
JS dependency Zero JS 12KB gzip
Performance Browser native JS calculation
Declarative CSS declarative JS imperative
Boundary flip position-fallback flip() middleware
Offset margin/anchor() offset() middleware
Arrow CSS pseudo-element SVG/HTML element
Virtual elements Not supported Supported
Dynamic content Auto-follow Needs update()
Browser support Chrome 125+ All browsers
SSR friendly Fully friendly Needs client JS

When You Still Need JS Libraries

Scenarios still requiring Floating UI:
1. Need to support older browsers (Safari < 17.5, Firefox < 131)
2. Need virtual element positioning (cursor position following)
3. Need complex middleware chains (autoUpdate, size, hide)
4. React/Vue declarative wrapper requirements

Scenarios where CSS Anchor works:
1. Tooltip, Dropdown, Popover and other standard popups
2. Only need to support modern browsers
3. Pursuing zero JS dependency
4. Performance-sensitive scenarios

Browser Compatibility and Polyfill

Support Status

Browser Anchor Positioning Popover API
Chrome 125+ ✅ ✅
Edge 125+ ✅ ✅
Safari 17.5+ ✅ ✅
Firefox 131+ ✅ ✅
iOS Safari 17.5+ ✅ ✅

Polyfill Solutions

<script src="https://cdn.jsdelivr.net/npm/@oddbird/css-anchor-positioning@0.3.0/dist/css-anchor-positioning.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/@oddbird/popover-polyfill@0.4.0/dist/popover.min.js"></script>

Combining with @starting-style for Popup Animations

[popover] {
  opacity: 0;
  transform: translateY(-8px) scale(0.95);
  transition: opacity 0.2s, transform 0.2s, display 0.2s allow-discrete;
  overlay: none;
}

[popover]:popover-open {
  opacity: 1;
  transform: translateY(0) scale(1);
  overlay: auto;
}

@starting-style {
  [popover]:popover-open {
    opacity: 0;
    transform: translateY(-8px) scale(0.95);
  }
}

Summary

  1. CSS Anchor Positioning is the ultimate solution for popup positioning — Zero JS, declarative, browser native
  2. Popover API makes popup toggling zero JS too — popovertarget attribute handles it in one line
  3. position-fallback solves boundary overflow — No more JS calculation for flipping
  4. All major browsers support it in 2026 — Safe to use in production

After 10 years, popup positioning has finally returned to where CSS belongs. This isn't an incremental improvement — it's a paradigm shift, just like Flexbox replacing float layouts.

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#CSS锚点定位#Anchor Positioning#Popover API#弹出层#CSS