diff --git a/assets/javascripts/discourse/api-initializers/class.js b/assets/javascripts/discourse/initializers/bbcode-class-styles.js
similarity index 77%
rename from assets/javascripts/discourse/api-initializers/class.js
rename to assets/javascripts/discourse/initializers/bbcode-class-styles.js
index 2ef1b19..2540b6f 100644
--- a/assets/javascripts/discourse/api-initializers/class.js
+++ b/assets/javascripts/discourse/initializers/bbcode-class-styles.js
@@ -1,7 +1,7 @@
 /**
  * @file Initializes and adds any custom user styles to the post
  */
-import { apiInitializer } from "discourse/lib/api";
+import { withPluginApi } from "discourse/lib/plugin-api";
 
 /**
  * Adds the inline styles for the style tag inside a given post
@@ -30,6 +30,14 @@ function addClassStyleCode(post) {
   });
 }
 
-export default apiInitializer((api) => {
-  api.decorateCookedElement(addClassStyleCode, { id: "add class style code" });
-});
+export default {
+  name: "bbcode-class-styles",
+  after: "inject-objects",
+  initialize() {
+    withPluginApi((api) => {
+      api.decorateCookedElement(addClassStyleCode, {
+        id: "add class style code",
+      });
+    });
+  },
+};
diff --git a/assets/javascripts/discourse/initializers/bbcode-css-upload-urls.js b/assets/javascripts/discourse/initializers/bbcode-css-upload-urls.js
new file mode 100644
index 0000000..a1de89d
--- /dev/null
+++ b/assets/javascripts/discourse/initializers/bbcode-css-upload-urls.js
@@ -0,0 +1,115 @@
+/**
+ * @file Resolves upload:// references left behind in bbcode-generated CSS by
+ * client-side cooking (e.g. the composer preview): the server resolves these
+ * at cook time, but the preview's markdown-it engine has no upload lookup, so
+ * it cooks the literal upload:// text through unresolved.
+ */
+import {
+  lookupCachedUploadUrl,
+  lookupUncachedUploadUrls,
+  MISSING,
+} from "pretty-text/upload-short-url";
+import { ajax } from "discourse/lib/ajax";
+import discourseDebounce from "discourse/lib/debounce";
+import { withPluginApi } from "discourse/lib/plugin-api";
+
+const UPLOAD_URL_REGEX = /url\(\s*(['"]?)(upload:\/\/[^'")]+)\1\s*\)/gi;
+
+let queuedShortUrls;
+let queuePromise;
+let queueResolve;
+
+function queuePop() {
+  lookupUncachedUploadUrls(queuedShortUrls, ajax).then(queueResolve);
+  queuedShortUrls = queueResolve = null;
+}
+
+// Batches short-url lookups across a debounce window (matching core's own
+// composer-preview resolver) so multiple decorated posts on the same page
+// share one request instead of firing one each.
+function resolveUncachedShortUrls(shortUrls) {
+  if (!queuedShortUrls) {
+    queuedShortUrls = [...shortUrls];
+    queuePromise = new Promise((resolve) => (queueResolve = resolve));
+    discourseDebounce(null, queuePop, 450);
+  } else {
+    queuedShortUrls.push(...shortUrls);
+  }
+  return queuePromise;
+}
+
+function cssOf(node) {
+  return node.tagName === "STYLE"
+    ? node.textContent
+    : node.getAttribute("style");
+}
+
+function setCssOf(node, css) {
+  if (node.tagName === "STYLE") {
+    node.textContent = css;
+  } else {
+    node.setAttribute("style", css);
+  }
+}
+
+/**
+ * @param {HTMLElement} post the post itself
+ */
+async function resolveCssUploadUrls(post) {
+  const nodes = [
+    ...post.querySelectorAll("style[data-rendered-class]"),
+    ...post.querySelectorAll("[style*='upload://']"),
+  ];
+
+  const shortUrls = new Set();
+  nodes.forEach((node) => {
+    for (const match of cssOf(node).matchAll(UPLOAD_URL_REGEX)) {
+      shortUrls.add(match[2]);
+    }
+  });
+
+  if (shortUrls.size === 0) {
+    return;
+  }
+
+  const uncached = [...shortUrls].filter(
+    (url) => !lookupCachedUploadUrl(url).url
+  );
+  if (uncached.length > 0) {
+    await resolveUncachedShortUrls(uncached);
+  }
+
+  const resolved = new Map();
+  shortUrls.forEach((url) => {
+    const { url: resolvedUrl } = lookupCachedUploadUrl(url);
+    if (resolvedUrl && resolvedUrl !== MISSING) {
+      resolved.set(url, resolvedUrl);
+    }
+  });
+
+  if (resolved.size === 0) {
+    return;
+  }
+
+  const replaced = (css) =>
+    css.replace(UPLOAD_URL_REGEX, (fullMatch, quote, shortUrl) => {
+      const url = resolved.get(shortUrl);
+      return url ? `url(${quote}${url}${quote})` : fullMatch;
+    });
+
+  nodes.forEach((node) => setCssOf(node, replaced(cssOf(node))));
+}
+
+export default {
+  name: "bbcode-css-upload-urls",
+  // must run after class-tag CSS is materialized into a live <style> tag, since
+  // this patches that derived node rather than the <template> it comes from
+  after: ["inject-objects", "bbcode-class-styles"],
+  initialize() {
+    withPluginApi((api) => {
+      api.decorateCookedElement(resolveCssUploadUrls, {
+        id: "resolve bbcode css upload urls",
+      });
+    });
+  },
+};
diff --git a/assets/stylesheets/common/containment.scss b/assets/stylesheets/common/containment.scss
new file mode 100644
index 0000000..003b894
--- /dev/null
+++ b/assets/stylesheets/common/containment.scss
@@ -0,0 +1,10 @@
+%bbcode-post-containment {
+  clip-path: border-box;
+  contain: layout;
+  position: relative;
+}
+
+.cooked,
+.d-editor-preview {
+  @extend %bbcode-post-containment;
+}
diff --git a/assets/stylesheets/common/index.scss b/assets/stylesheets/common/index.scss
index 782fa6e..c6b2ce5 100644
--- a/assets/stylesheets/common/index.scss
+++ b/assets/stylesheets/common/index.scss
@@ -4,6 +4,7 @@
 @import "blockquote";
 @import "border-bg";
 @import "check";
+@import "containment";
 @import "divide";
 @import "fieldset";
 @import "heightrestrict";
@@ -26,7 +27,3 @@
 @import "spoiler";
 @import "tabs";
 @import "textmessage";
-
-.d-editor-preview {
-  position: relative;
-}
diff --git a/lib/bb_code/css_hotlinked_media.rb b/lib/bb_code/css_hotlinked_media.rb
new file mode 100644
index 0000000..2211e29
--- /dev/null
+++ b/lib/bb_code/css_hotlinked_media.rb
@@ -0,0 +1,125 @@
+# frozen_string_literal: true
+
+# Rehosts external images referenced through bbcode-generated CSS (the [class]/
+# [animation] style block, and raw style="" attributes from [div]/[tab]/
+# [accordion]) the same way core rehosts <img src>: prepend onto
+# HotlinkedMedia.extract_candidates so core's own PullHotlinkedImages job
+# downloads and books them, then rewrite the cooked doc to point at the local
+# upload once downloaded.
+module ::BbCode
+  module CssHotlinkedMedia
+    STYLE_TEMPLATE_SELECTOR = 'template[data-bbcode-plus="class"]'
+    URL_FN_REGEX = /url\(\s*(['"]?)([^'")]+)\1\s*\)/i
+
+    # Duck-types just enough of a Nokogiri node to satisfy
+    # HotlinkedMedia.download_src_for (node["src"]) and callers that branch on
+    # node.name, e.g. plugins/chat's job.
+    CandidateNode =
+      Struct.new(:url) do
+        def [](key)
+          url if key == "src"
+        end
+
+        def name
+          "bbcode-css-url"
+        end
+      end
+
+    module ExtendExtractCandidates
+      def extract_candidates(html)
+        doc = html.is_a?(Nokogiri::XML::Node) ? html : Nokogiri::HTML5.fragment(html)
+        super(doc).to_a + ::BbCode::CssHotlinkedMedia.extract_candidates(doc)
+      end
+    end
+
+    # Keeps post.raw in sync the same way core does for img src/href: once a
+    # CSS url() has a downloaded PostHotlinkedMedia record, point the raw
+    # source at it too (as an upload:// short-url, which rewrite_css above
+    # already knows how to resolve back into a real URL on every future cook).
+    module RewriteRawCssUrls
+      def replace_hotlinked_image_urls(raw:, &blk)
+        rewritten = super
+
+        rewritten.gsub(URL_FN_REGEX) do |match|
+          quote = Regexp.last_match(1)
+          src = Regexp.last_match(2)
+          upload = blk.call(src)
+          upload ? "url(#{quote}#{upload.short_url}#{quote})" : match
+        end
+      end
+    end
+
+    def self.each_css_node(doc)
+      doc.css(STYLE_TEMPLATE_SELECTOR).each { |node| yield node, node.text }
+      doc.css("[style]").each { |node| yield node, node["style"] }
+    end
+
+    # Remote urls referenced from bbcode CSS in +doc_or_html+, as candidate
+    # nodes for ::HotlinkedMedia.extract_candidates to append to its own list.
+    def self.extract_candidates(doc_or_html)
+      doc =
+        doc_or_html.is_a?(Nokogiri::XML::Node) ? doc_or_html : Nokogiri::HTML5.fragment(doc_or_html)
+
+      urls = Set.new
+      each_css_node(doc) { |_node, css| css.scan(URL_FN_REGEX) { |_, url| urls << url } }
+      urls
+        .reject { |url| url.start_with?("upload://") }
+        .select { |url| ::HotlinkedMedia.remote_src?(url) }
+        .map { |url| CandidateNode.new(url) }
+    end
+
+    # Rewrites +doc+ in place: upload:// references resolve unconditionally
+    # (the same scheme core resolves for img src/a href), and hotlinked
+    # external urls resolve using already-downloaded PostHotlinkedMedia
+    # records on +post+. Returns whether anything changed.
+    def self.rewrite_doc!(doc, post)
+      return false unless doc.at_css("#{STYLE_TEMPLATE_SELECTOR}, [style]")
+
+      hotlinked_map = post.post_hotlinked_media.includes(:upload).index_by(&:url)
+      changed = false
+
+      each_css_node(doc) do |node, css|
+        new_css = rewrite_css(css, hotlinked_map, post)
+        next if new_css == css
+
+        if node.name == "template"
+          node.content = new_css
+        else
+          node["style"] = new_css
+        end
+        changed = true
+      end
+
+      changed
+    end
+
+    def self.rewrite_css(css, hotlinked_map, post)
+      css.gsub(URL_FN_REGEX) do |match|
+        quote = Regexp.last_match(1)
+        url = Regexp.last_match(2)
+        upload = resolve_upload(url, hotlinked_map)
+
+        if upload
+          cooked_url = UrlHelper.cook_url(upload.url, secure: post.should_secure_uploads?)
+          "url(#{quote}#{cooked_url}#{quote})"
+        else
+          match
+        end
+      end
+    end
+
+    # The upload a CSS url() should resolve to: either an upload:// short-url
+    # (the same scheme core resolves for img src/a href), or an external url
+    # already hotlinked and downloaded onto +post+.
+    def self.resolve_upload(url, hotlinked_map)
+      if url.start_with?("upload://")
+        sha1 = Upload.sha1_from_short_url(url)
+        Upload.find_by(sha1: sha1) if sha1.present?
+      else
+        record = hotlinked_map[PostHotlinkedMedia.normalize_src(url)]
+        record.upload if record&.downloaded?
+      end
+    end
+    private_class_method :resolve_upload
+  end
+end
diff --git a/plugin.rb b/plugin.rb
index 48e8462..bbf9f4e 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -27,6 +27,7 @@ module ::BbCode
 end
 
 require_relative "lib/bb_code/engine"
+require_relative "lib/bb_code/css_hotlinked_media"
 
 after_initialize do
   # Code which should run after Rails has finished booting
@@ -37,6 +38,11 @@ module ::BbCode
     MessageBus.subscribe(::BbCode::ENGINE_RESET_CHANNEL) { PrettyText.reset_context }
   end
 
+  ::HotlinkedMedia.singleton_class.prepend(::BbCode::CssHotlinkedMedia::ExtendExtractCandidates)
+  ::InlineUploads.singleton_class.prepend(::BbCode::CssHotlinkedMedia::RewriteRawCssUrls)
+
+  on(:post_process_cooked) { |doc, post| ::BbCode::CssHotlinkedMedia.rewrite_doc!(doc, post) }
+
   # overrides the default normalize_whitespaces function in discourse/lib/text_cleaner.rb
   # adds discourse_normalize_whitespace setting (defaults to false)
   # when true, normalize_whitespace runs as normal