diff --git a/Source/WebCore/html/HTMLAttachmentElement.cpp b/Source/WebCore/html/HTMLAttachmentElement.cpp
index 1accd1c79b42..ad3b37422f99 100644
--- a/Source/WebCore/html/HTMLAttachmentElement.cpp
+++ b/Source/WebCore/html/HTMLAttachmentElement.cpp
@@ -114,6 +114,18 @@ static const AtomString& attachmentPreviewIdentifier()
     return identifier;
 }
 
+static const AtomString& attachmentProgressIdentifier()
+{
+    static MainThreadNeverDestroyed<const AtomString> identifier("attachment-progress"_s);
+    return identifier;
+}
+
+static const AtomString& attachmentProgressCircleIdentifier()
+{
+    static MainThreadNeverDestroyed<const AtomString> identifier("attachment-progress-circle"_s);
+    return identifier;
+}
+
 static const AtomString& attachmentInformationAreaIdentifier()
 {
     static MainThreadNeverDestroyed<const AtomString> identifier("attachment-information-area"_s);
@@ -226,6 +238,11 @@ void HTMLAttachmentElement::ensureModernShadowTree(ShadowRoot& root)
     m_innerLegacyAttachment->setIdAttribute(attachmentPreviewIdentifier());
     previewArea->appendChild(*m_innerLegacyAttachment);
 
+    m_progressElement = createContainedElement<HTMLDivElement>(previewArea, attachmentProgressIdentifier());
+    updateProgress(attributeWithoutSynchronization(progressAttr));
+
+    createContainedElement<HTMLDivElement>(*m_progressElement, attachmentProgressCircleIdentifier());
+
     auto informationArea = createContainedElement<HTMLDivElement>(*m_containerElement, attachmentInformationAreaIdentifier());
 
     m_informationBlock = createContainedElement<HTMLDivElement>(informationArea, attachmentInformationBlockIdentifier());
@@ -275,6 +292,21 @@ class AttachmentSaveEventListener final : public EventListener {
     WeakPtr<HTMLAttachmentElement, WeakPtrImplWithEventTargetData> m_attachment;
 };
 
+void HTMLAttachmentElement::updateProgress(const AtomString& progress)
+{
+    if (!m_progressElement)
+        return;
+
+    bool validProgress = false;
+    float value = progress.toFloat(&validProgress);
+    if (validProgress && std::isfinite(value)) {
+        m_progressElement->setAttributeWithoutSynchronization(styleAttr, makeAtomString("--progress: ", (value < 0.0) ? "0"_s : (value > 1.0) ? "1"_s : progress));
+        return;
+    }
+
+    m_progressElement->setAttributeWithoutSynchronization(styleAttr, "display: none;"_s);
+}
+
 void HTMLAttachmentElement::updateSaveButton(bool show)
 {
     if (!show) {
@@ -425,10 +457,12 @@ RefPtr<HTMLImageElement> HTMLAttachmentElement::enclosingImageElement() const
 
 void HTMLAttachmentElement::parseAttribute(const QualifiedName& name, const AtomString& value)
 {
-    if (name == actionAttr || name == progressAttr || name == subtitleAttr() || name == titleAttr || name == typeAttr) {
+    if (name == actionAttr || name == subtitleAttr() || name == titleAttr || name == typeAttr) {
         if (m_innerLegacyAttachment)
             m_innerLegacyAttachment->setAttributeWithoutSynchronization(name, value);
         invalidateRendering();
+    } else if (name == progressAttr && m_implementation == Implementation::Legacy) {
+        invalidateRendering();
     }
 
     HTMLElement::parseAttribute(name, value);
@@ -442,7 +476,9 @@ void HTMLAttachmentElement::parseAttribute(const QualifiedName& name, const Atom
     } else if (name == subtitleAttr()) {
         if (m_subtitleElement)
             m_subtitleElement->setTextContent(String(value.string()));
-    } else if (name == saveAttr())
+    } else if (name == progressAttr)
+        updateProgress(value);
+    else if (name == saveAttr())
         updateSaveButton(!value.isNull());
 
 #if ENABLE(SERVICE_CONTROLS)
diff --git a/Source/WebCore/html/HTMLAttachmentElement.h b/Source/WebCore/html/HTMLAttachmentElement.h
index e90c5ba7321b..e76d993b7ef1 100644
--- a/Source/WebCore/html/HTMLAttachmentElement.h
+++ b/Source/WebCore/html/HTMLAttachmentElement.h
@@ -98,6 +98,7 @@ class HTMLAttachmentElement final : public HTMLElement {
 
     void didAddUserAgentShadowRoot(ShadowRoot&) final;
     void ensureModernShadowTree(ShadowRoot&);
+    void updateProgress(const AtomString&);
     void updateSaveButton(bool);
 
     RenderPtr<RenderElement> createElementRenderer(RenderStyle&&, const RenderTreePosition&) final;
@@ -126,6 +127,7 @@ class HTMLAttachmentElement final : public HTMLElement {
 
     RefPtr<HTMLAttachmentElement> m_innerLegacyAttachment;
     RefPtr<HTMLElement> m_containerElement;
+    RefPtr<HTMLElement> m_progressElement;
     RefPtr<HTMLElement> m_informationBlock;
     RefPtr<HTMLElement> m_actionTextElement;
     RefPtr<HTMLElement> m_titleElement;
diff --git a/Source/WebCore/html/shadow/attachmentElementShadow.css b/Source/WebCore/html/shadow/attachmentElementShadow.css
index f98b63688614..a307439a56bc 100644
--- a/Source/WebCore/html/shadow/attachmentElementShadow.css
+++ b/Source/WebCore/html/shadow/attachmentElementShadow.css
@@ -80,6 +80,25 @@ attachment#attachment-preview {
     overflow: hidden;
 }
 
+div#attachment-progress {
+    grid-row: 1;
+    grid-column: 1;
+    width: 100%;
+    aspect-ratio: 1;
+    border-radius: 50%;
+    color: -apple-system-secondary-label;
+    background: conic-gradient(currentcolor calc(var(--progress) * 100%), transparent 0);
+}
+
+/* FIXME: Move the border into attachment-progress above, and remove this div, when rdar://107621207 is fixed (currently it produces artifacts at the edges). */
+div#attachment-progress-circle {
+    width: 100%;
+    aspect-ratio: 1;
+    border-radius: 50%;
+    border: 4px solid currentcolor;
+    box-sizing: border-box;
+}
+
 div#attachment-information-area {
     grid-row: 1;
     grid-column: 2;