diff --git a/reader.html b/reader.html
index 8820ea3..69e5634 100644
--- a/reader.html
+++ b/reader.html
@@ -237,6 +237,20 @@ body {
     background-repeat: no-repeat;
     background-image: url('data:image/svg+xml;utf8,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%2224%22%20height%3D%2224%22%3E%3Ccircle%20cx%3D%2212%22%20cy%3D%2212%22%20r%3D%223%22%2F%3E%3C%2Fsvg%3E');
 }
+.popover {
+    background: Canvas;
+    color: CanvasText;
+    border-radius: 6px;
+    box-shadow: 0 0 0 1px rgba(0, 0, 0, .2), 0 0 16px rgba(0, 0, 0, .1), 0 0 32px rgba(0, 0, 0, .1);
+}
+.popover-arrow-down {
+    fill: Canvas;
+    filter: drop-shadow(0 -1px 0 rgba(0, 0, 0, .2));
+}
+.popover-arrow-up {
+    fill: Canvas;
+    filter: drop-shadow(0 1px 0 rgba(0, 0, 0, .2));
+}
 </style>
 <input type="file" id="file-input" hidden>
 <div id="drop-target" class="filter">
diff --git a/reader.js b/reader.js
index 86ee8b2..59d07ee 100644
--- a/reader.js
+++ b/reader.js
@@ -2,6 +2,7 @@
 import { View } from './view.js'
 import { createTOCView } from './ui/tree.js'
 import { createMenu } from './ui/menu.js'
+import { createPopover } from './ui/popover.js'
 
 const { ZipReader, BlobReader, TextWriter, BlobWriter } = zip
 zip.configure({ useWebWorkers: false })
@@ -119,6 +120,30 @@ const emit = obj => {
             if (tocItem?.href) setCurrentHref?.(tocItem.href)
             break
         }
+        case 'reference': {
+            const { content, pos: { point, dir } } = obj
+            const iframe = document.createElement('iframe')
+            iframe.sandbox = 'allow-same-origin'
+            iframe.srcdoc = content
+            iframe.onload = () => {
+                const doc = iframe.contentDocument
+                doc.documentElement.style.colorScheme = 'light dark'
+                doc.body.style.margin = '18px'
+            }
+            Object.assign(iframe.style, {
+                border: '0',
+                width: '100%',
+                height: '100%',
+            })
+            const { popover, arrow, overlay } = createPopover(300, 250, point, dir)
+            overlay.style.zIndex = 3
+            popover.style.zIndex = 3
+            arrow.style.zIndex = 3
+            popover.append(iframe)
+            document.body.append(overlay)
+            document.body.append(popover)
+            document.body.append(arrow)
+        }
     }
 }
 
diff --git a/ui/popover.js b/ui/popover.js
new file mode 100644
index 0000000..5b99618
--- /dev/null
+++ b/ui/popover.js
@@ -0,0 +1,61 @@
+const clamp = (min, max, x) => Math.min(max, Math.max(min, x))
+
+const arrowHeight = 10
+const arrowWidth = 20
+const radius = 6
+
+const createSVGElement = tag =>
+    document.createElementNS('http://www.w3.org/2000/svg', tag)
+
+const createArrow = down => {
+    const h = arrowHeight + 1
+    const svg = createSVGElement('svg')
+    svg.setAttribute('width', arrowWidth)
+    svg.setAttribute('height', arrowHeight)
+    const polygon = createSVGElement('polygon')
+    polygon.setAttribute('points', down
+        ? `0 ${h}, ${arrowWidth / 2} 0, ${arrowWidth} ${h}`
+        : `0 0, ${arrowWidth / 2} ${h}, ${arrowWidth} 0`)
+    svg.classList.add(down ? 'popover-arrow-down' : 'popover-arrow-up')
+    svg.append(polygon)
+    return svg
+}
+
+export const createPopover = (width, height, { x, y }, dir) => {
+    const down = dir === 'down'
+    const fullHeight = height + arrowHeight
+    const overlay = document.createElement('div')
+    Object.assign(overlay.style, {
+        position: 'absolute',
+        left: '0',
+        top: '0',
+        width: '100vw',
+        height: '100vh',
+    })
+    const arrow = createArrow(down)
+    Object.assign(arrow.style, {
+        position: 'absolute',
+        left: `${clamp(radius, window.innerWidth - arrowWidth - radius,
+            x - arrowWidth / 2)}px`,
+        top: `${clamp(0, window.innerHeight - arrowHeight,
+            down ? y : y - arrowHeight)}px`,
+    })
+    const popover = document.createElement('div')
+    popover.classList.add('popover')
+    Object.assign(popover.style, {
+        position: 'absolute',
+        boxSizing: 'border-box',
+        overflow: 'hidden',
+        left: `${clamp(0, window.innerWidth - width, x - width / 2)}px`,
+        top: `${clamp(0, window.innerHeight - fullHeight,
+            down ? y + arrowHeight : y - fullHeight)}px`,
+        width: `${width}px`,
+        height: `${height}px`,
+    })
+    overlay.addEventListener('click', () => {
+        overlay.parentNode.removeChild(overlay)
+        arrow.parentNode.removeChild(arrow)
+        popover.parentNode.removeChild(popover)
+    })
+    return { popover, arrow, overlay }
+}