From 9473a49837ef484a9b0caed50b9f3bf03b7ee175 Mon Sep 17 00:00:00 2001
From: Tayeb Chlyah <tayebchlyah@gmail.com>
Date: Sat, 9 May 2026 07:03:33 +0400
Subject: [PATCH] feat(client, server) #11: add vega + vega-lite renderer

Design spec: docs/superpowers/specs/2026-05-09-vega-renderer-design.md
---
 client/components/editor/editor-markdown.vue  |   5 +
 client/components/vega/hydrate.js             | 148 +++++++++++
 client/scss/app.scss                          |   1 +
 client/scss/components/vega.scss              |  35 +++
 client/themes/default/components/page.vue     |   4 +
 .../specs/2026-05-09-vega-renderer-design.md  | 230 ++++++++++++++++++
 .../rendering/html-vega-lite/definition.yml   |   8 +
 .../rendering/html-vega-lite/renderer.js      |   8 +
 .../rendering/html-vega/definition.yml        |   8 +
 .../modules/rendering/html-vega/renderer.js   |   8 +
 server/test/rendering/vega.test.js            |  56 +++++
 11 files changed, 511 insertions(+)
 create mode 100644 client/components/vega/hydrate.js
 create mode 100644 client/scss/components/vega.scss
 create mode 100644 docs/superpowers/specs/2026-05-09-vega-renderer-design.md
 create mode 100644 server/modules/rendering/html-vega-lite/definition.yml
 create mode 100644 server/modules/rendering/html-vega-lite/renderer.js
 create mode 100644 server/modules/rendering/html-vega/definition.yml
 create mode 100644 server/modules/rendering/html-vega/renderer.js
 create mode 100644 server/test/rendering/vega.test.js

diff --git a/client/components/editor/editor-markdown.vue b/client/components/editor/editor-markdown.vue
index fad1cda6..4fc1220d 100644
--- a/client/components/editor/editor-markdown.vue
+++ b/client/components/editor/editor-markdown.vue
@@ -228,6 +228,9 @@ import Prism from 'prismjs'
 // Mermaid
 import mermaid from 'mermaid'
 
+// Vega
+import { hydrateVega } from '../vega/hydrate'
+
 // Helpers
 import katexHelper from './common/katex'
 import tabsetHelper from './markdown/tabset'
@@ -419,6 +422,7 @@ export default {
       if (newValue && !oldValue) {
         this.$nextTick(() => {
           this.renderMermaidDiagrams()
+          hydrateVega(this.$refs.editorPreview, { darkMode: this.$vuetify.theme.dark })
           Prism.highlightAllUnder(this.$refs.editorPreview)
           Array.from(this.$refs.editorPreview.querySelectorAll('pre.line-numbers')).forEach(pre => pre.classList.add('prismjs'))
         })
@@ -472,6 +476,7 @@ export default {
       this.$nextTick(() => {
         tabsetHelper.format()
         this.renderMermaidDiagrams()
+        hydrateVega(this.$refs.editorPreview, { darkMode: this.$vuetify.theme.dark })
         Prism.highlightAllUnder(this.$refs.editorPreview)
         Array.from(this.$refs.editorPreview.querySelectorAll('pre.line-numbers')).forEach(pre => pre.classList.add('prismjs'))
         this.scrollSync(this.cm)
diff --git a/client/components/vega/hydrate.js b/client/components/vega/hydrate.js
new file mode 100644
index 00000000..1872501d
--- /dev/null
+++ b/client/components/vega/hydrate.js
@@ -0,0 +1,148 @@
+const VEGA_SPEC_ATTR = 'data-vega-spec'
+
+const VEGA_VERSION = '6.2.0'
+const VEGA_LITE_VERSION = '6.4.3'
+const VEGA_EMBED_VERSION = '7.1.0'
+
+const CDN_BASE = 'https://cdn.jsdelivr.net/npm'
+const VEGA_SCRIPTS = [
+  { url: `${CDN_BASE}/vega@${VEGA_VERSION}/build/vega.min.js`, global: 'vega' },
+  { url: `${CDN_BASE}/vega-lite@${VEGA_LITE_VERSION}/build/vega-lite.min.js`, global: 'vegaLite' },
+  { url: `${CDN_BASE}/vega-embed@${VEGA_EMBED_VERSION}/build/vega-embed.min.js`, global: 'vegaEmbed' }
+]
+
+let vegaEmbedPromise = null
+
+function loadScript(url) {
+  return new Promise((resolve, reject) => {
+    const existing = document.querySelector(`script[data-vega-cdn="${url}"]`)
+    if (existing) {
+      if (existing.dataset.loaded === 'true') {
+        resolve()
+        return
+      }
+      existing.addEventListener('load', () => resolve())
+      existing.addEventListener('error', () => reject(new Error(`Failed to load ${url}`)))
+      return
+    }
+    const script = document.createElement('script')
+    script.src = url
+    script.async = false
+    script.dataset.vegaCdn = url
+    script.addEventListener('load', () => {
+      script.dataset.loaded = 'true'
+      resolve()
+    })
+    script.addEventListener('error', () => reject(new Error(`Failed to load ${url}`)))
+    document.head.appendChild(script)
+  })
+}
+
+async function loadVegaEmbed() {
+  if (vegaEmbedPromise) {
+    return vegaEmbedPromise
+  }
+  vegaEmbedPromise = (async () => {
+    for (const { url, global } of VEGA_SCRIPTS) {
+      await loadScript(url)
+      if (typeof window[global] === 'undefined') {
+        throw new Error(`Loaded ${url} but ${global} global is undefined`)
+      }
+    }
+    return window.vegaEmbed
+  })().catch(err => {
+    vegaEmbedPromise = null
+    throw err
+  })
+  return vegaEmbedPromise
+}
+
+function readSpecText(el) {
+  if (el.hasAttribute(VEGA_SPEC_ATTR)) {
+    return el.getAttribute(VEGA_SPEC_ATTR)
+  }
+  const text = el.textContent
+  el.setAttribute(VEGA_SPEC_ATTR, text)
+  return text
+}
+
+function renderError(el, message) {
+  const sourceText = el.getAttribute(VEGA_SPEC_ATTR) || el.textContent
+  el.classList.add('vega-error')
+  el.replaceChildren()
+
+  const heading = document.createElement('strong')
+  heading.textContent = 'Vega render error'
+
+  const messageEl = document.createElement('p')
+  messageEl.textContent = message
+
+  const sourceEl = document.createElement('pre')
+  sourceEl.textContent = sourceText
+
+  el.appendChild(heading)
+  el.appendChild(messageEl)
+  el.appendChild(sourceEl)
+}
+
+function normalizeRawCodeBlocks(root) {
+  // Some pipelines (e.g., editor preview) leave fenced vega blocks as
+  // `<pre><code class="language-vega">…</code></pre>`. Convert those to the
+  // same `<div class="vega|vega-lite">…</div>` shape produced by the server-
+  // side renderer, so the rest of the hydrator works uniformly.
+  const codeNodes = root.querySelectorAll('pre > code.language-vega, pre > code.language-vega-lite')
+  codeNodes.forEach(code => {
+    const lang = code.classList.contains('language-vega-lite') ? 'vega-lite' : 'vega'
+    const div = document.createElement('div')
+    div.className = lang
+    div.textContent = code.textContent
+    const pre = code.parentElement
+    pre.parentNode.replaceChild(div, pre)
+  })
+}
+
+export async function hydrateVega(rootEl, { darkMode = false } = {}) {
+  const root = rootEl || document
+  normalizeRawCodeBlocks(root)
+  const elements = Array.from(root.querySelectorAll('.vega, .vega-lite'))
+  if (elements.length === 0) {
+    return
+  }
+
+  // Capture spec text before any DOM mutation so re-runs stay idempotent.
+  elements.forEach(el => readSpecText(el))
+
+  let embed
+  try {
+    embed = await loadVegaEmbed()
+  } catch (err) {
+    elements.forEach(el => renderError(el, `Failed to load Vega library: ${err.message}`))
+    // eslint-disable-next-line no-console
+    console.error('[vega] failed to load vega-embed from CDN', err)
+    return
+  }
+
+  const theme = darkMode ? 'dark' : undefined
+
+  await Promise.all(elements.map(async el => {
+    const specText = readSpecText(el)
+    const mode = el.classList.contains('vega-lite') ? 'vega-lite' : 'vega'
+
+    let spec
+    try {
+      spec = JSON.parse(specText)
+    } catch (err) {
+      renderError(el, `Invalid JSON: ${err.message}`)
+      return
+    }
+
+    el.replaceChildren()
+    el.classList.remove('vega-error')
+
+    try {
+      await embed(el, spec, { mode, actions: false, theme })
+    } catch (err) {
+      renderError(el, err.message || String(err))
+    }
+  }))
+}
diff --git a/client/scss/app.scss b/client/scss/app.scss
index 566934ec..d4cb2610 100644
--- a/client/scss/app.scss
+++ b/client/scss/app.scss
@@ -15,6 +15,7 @@
 @import 'components/v-dialog';
 @import 'components/v-form';
 @import 'components/v-tabs';
+@import 'components/vega';
 
 // @import '../libs/twemoji/twemoji-awesome';
 // @import '../libs/prism/prism.css';
diff --git a/client/scss/components/vega.scss b/client/scss/components/vega.scss
new file mode 100644
index 00000000..850952b5
--- /dev/null
+++ b/client/scss/components/vega.scss
@@ -0,0 +1,35 @@
+.vega,
+.vega-lite {
+  display: block;
+  margin: 1rem 0;
+  overflow-x: auto;
+}
+
+.vega-error {
+  border: 1px solid #c62828;
+  background: rgba(198, 40, 40, 0.05);
+  color: #c62828;
+  padding: 12px 16px;
+  border-radius: 4px;
+  font-size: 0.9rem;
+
+  strong {
+    display: block;
+    margin-bottom: 4px;
+  }
+
+  p {
+    margin: 4px 0 8px;
+  }
+
+  pre {
+    background: rgba(0, 0, 0, 0.05);
+    padding: 8px;
+    border-radius: 2px;
+    overflow-x: auto;
+    font-size: 0.8rem;
+    margin: 0;
+    white-space: pre-wrap;
+    word-break: break-word;
+  }
+}
diff --git a/client/themes/default/components/page.vue b/client/themes/default/components/page.vue
index 76d6baf1..8541238e 100644
--- a/client/themes/default/components/page.vue
+++ b/client/themes/default/components/page.vue
@@ -382,6 +382,7 @@ import Tabset from './tabset.vue'
 import NavSidebar from './nav-sidebar.vue'
 import Prism from 'prismjs'
 import mermaid from 'mermaid'
+import { hydrateVega } from '../../../components/vega/hydrate'
 import { get, sync } from 'vuex-pathify'
 import _ from 'lodash'
 import ClipboardJS from 'clipboard'
@@ -645,6 +646,9 @@ export default {
       theme: this.$vuetify.theme.dark ? `dark` : `default`
     })
 
+    // -> Render Vega / Vega-Lite charts
+    hydrateVega(this.$refs.container, { darkMode: this.$vuetify.theme.dark })
+
     // -> Handle anchor scrolling
     if (window.location.hash && window.location.hash.length > 1) {
       if (document.readyState === 'complete') {
diff --git a/docs/superpowers/specs/2026-05-09-vega-renderer-design.md b/docs/superpowers/specs/2026-05-09-vega-renderer-design.md
new file mode 100644
index 00000000..1f22bbae
--- /dev/null
+++ b/docs/superpowers/specs/2026-05-09-vega-renderer-design.md
@@ -0,0 +1,230 @@
+# Vega + Vega-Lite Renderer — Design Spec
+
+**Date:** 2026-05-09
+**Status:** Approved
+**Author:** Tayeb Chlyah
+
+---
+
+## Goal
+
+Add support for [Vega](https://vega.github.io/vega/) and [Vega-Lite](https://vega.github.io/vega-lite/) charts in Wiki.js pages via fenced code blocks, using the official `vega-embed` library. Charts render in both the live page view and the markdown editor preview.
+
+---
+
+## Context
+
+Wiki.js already supports several embedded visualization syntaxes via the same pattern: a server-side rendering module rewrites highlighted fenced blocks into a placeholder `<div>`, and the client mounts a JS library on those divs after the page loads.
+
+Reference modules:
+- `server/modules/rendering/html-mermaid/` — minimal renderer (8-line `init`).
+- Hydration in `client/themes/default/components/page.vue` (live view) and `client/components/editor/editor-markdown.vue` (editor preview).
+
+This spec follows that pattern, with deviations noted where Vega's characteristics differ from Mermaid.
+
+---
+
+## Markdown Syntax
+
+Two fenced-code languages, mapped to two engines:
+
+````markdown
+```vega
+{ ...full Vega spec... }
+```
+
+```vega-lite
+{ ...Vega-Lite spec... }
+```
+````
+
+The fence language unambiguously selects the engine. No auto-detection from `$schema`. Trailing whitespace and surrounding whitespace ignored. Spec body is JSON.
+
+---
+
+## Architecture
+
+### Server modules
+
+Two new rendering modules, modeled on `html-mermaid`:
+
+- `server/modules/rendering/html-vega/`
+  - `definition.yml`: `key: htmlVega`, `dependsOn: htmlCore`, `enabledDefault: true`, no props.
+  - `renderer.js`: matches `pre.prismjs > code.language-vega`, replaces parent `<pre>` with `<div class="vega">{spec text}</div>`.
+- `server/modules/rendering/html-vega-lite/`
+  - Same structure, key `htmlVegaLite`, class `vega-lite`, language `language-vega-lite`.
+
+Modules are independent — each can be enabled/disabled separately.
+
+### Client hydration
+
+Single shared helper module (`client/components/vega/hydrate.js`) exporting:
+
+```
+hydrateVega(rootEl, { darkMode })
+```
+
+Behavior:
+
+1. Query `rootEl.querySelectorAll('.vega, .vega-lite')`.
+2. If empty, return immediately (avoids paying the script-load cost on pages without charts).
+3. Lazily inject three `<script>` tags from a public CDN (jsdelivr) in order: `vega`, `vega-lite`, `vega-embed`. Each script registers a global (`window.vega`, `window.vegaLite`, `window.vegaEmbed`). Versions are pinned in the helper module.
+4. The load promise is cached at module scope so subsequent calls reuse the same loaded scripts.
+5. For each matched element:
+   - On first hydration, copy the element's text content into a `data-vega-spec` attribute (for idempotent re-runs).
+   - Read spec text from `data-vega-spec`, `JSON.parse`.
+   - Determine mode: `'vega-lite'` if element has `vega-lite` class, else `'vega'`.
+   - Call `window.vegaEmbed(el, spec, { mode, actions: false, theme: darkMode ? 'dark' : undefined })`.
+6. On any failure (parse error, embed rejection, script load failure), replace the element's content with an inline error box (see Error Handling).
+
+### Hydration call sites
+
+Two integrations:
+
+- **Live page view:** `client/themes/default/components/page.vue` mounted hook, alongside the existing Mermaid initialization. Passes `darkMode = this.$vuetify.theme.dark`.
+- **Editor preview:** `client/components/editor/editor-markdown.vue` preview render path. Hydration runs after each debounced markdown re-render. Because the preview pane innerHTML is replaced on each render, hydration must re-find the new elements each time. The CDN load promise is cached at module scope, so subsequent runs only pay the embed cost.
+
+### Markup normalization
+
+The two call sites produce different DOM shapes for vega blocks:
+
+- **Live page view:** the server-side `html-vega` / `html-vega-lite` renderers have already rewritten fenced code blocks to `<div class="vega">…</div>` / `<div class="vega-lite">…</div>` before the HTML reaches the browser.
+- **Editor preview:** the in-browser `markdown-it` instance produces the default fenced-block markup `<pre><code class="language-vega">…</code></pre>` (or `language-vega-lite`). The server-side renderer does not run on this path.
+
+The hydrator handles both shapes by running a `normalizeRawCodeBlocks` step at the start: any `pre > code.language-vega` / `pre > code.language-vega-lite` is replaced with the equivalent `<div class="vega">` / `<div class="vega-lite">` (using `textContent` to copy the spec text, so no HTML is ever injected). After normalization, the rest of the hydrator only deals with the unified div shape.
+
+This keeps the editor's `markdown-it` configuration untouched — earlier attempts to short-circuit `markdown-it`'s `highlight()` for vega languages failed because `markdown-it` wraps any non-`<pre>` highlight return value in `<pre><code class="language-X">`, defeating the trick.
+
+---
+
+## Bundle Strategy
+
+Vega libraries are **lazy-loaded from a public CDN** (jsdelivr) at runtime. No Vega code is bundled into the Wiki.js webpack output.
+
+Rationale:
+- Bundling was attempted first but was abandoned: Vega 6.x / Vega-Lite 6.x / vega-embed 7.x are ESM-only with `exports`-field package metadata and use modern syntax (optional chaining, import attributes) that the existing webpack 4 + babel + terser stack does not support without a substantial upgrade.
+- Combined Vega + Vega-Lite + vega-embed is roughly 700 KB gzipped — too large to eagerly include in the main bundle even if bundling were straightforward.
+- The CDN approach keeps the cost off pages that don't use charts and avoids the webpack 4 incompatibility entirely.
+
+Tradeoffs accepted (explicit user decision after bundling failed):
+- **Airgapped / on-prem deployments without internet egress will not render Vega charts.** The error box will display a "Failed to load Vega library" message.
+- A `Content-Security-Policy` configured to restrict `script-src` must allow `cdn.jsdelivr.net` (or the equivalent CDN domain) for chart pages.
+- Third-party CDN is now a runtime dependency. Outages on jsdelivr block chart rendering.
+
+The first vega block on a page incurs a one-time CDN load delay (roughly 1–2 seconds depending on network). Subsequent blocks reuse the cached script.
+
+---
+
+## Dependencies
+
+- **No new package.json dependencies.** Vega libraries are loaded at runtime from jsdelivr.
+- Versions of `vega`, `vega-lite`, and `vega-embed` are pinned as constants inside `client/components/vega/hydrate.js`.
+- Version bumps require editing the helper module and rebuilding (no `yarn upgrade` flow).
+- No additional server-side dependencies.
+
+---
+
+## Theming
+
+Pass `theme` to `vega-embed`:
+- `'dark'` when `$vuetify.theme.dark` is true.
+- `undefined` (vega-embed default) otherwise.
+
+Theme is read once at hydration time. Toggling dark/light mode after a chart renders does **not** re-render the chart — the page must be reloaded for the new palette to apply. This matches existing Mermaid behavior. Live theme reactivity is explicitly out of scope.
+
+---
+
+## Security Model
+
+Trusted-editor model — same as Mermaid and KaTeX:
+
+- Specs are passed straight through to `vega-embed` without sanitization.
+- Remote `data.url` references are allowed (vega-embed will fetch them at render time).
+- Vega expression parser is enabled (default vega-embed behavior).
+
+Rationale: page editors are already trusted with raw HTML and other dynamic content via existing renderers. Sandboxing Vega specs would block common, legitimate use cases (loading a CSV from a URL, expression-based encodings) without addressing a meaningfully different threat surface than what existing renderers already accept.
+
+This decision is recorded so future work can revisit if the trust model changes (e.g., introducing untrusted contributors).
+
+---
+
+## Error Handling
+
+All failure modes surface a visible inline error box, replacing the chart container's content. The hydrator catches:
+
+1. JSON parse failure on the spec text.
+2. `vega-embed` promise rejection (compile errors, runtime errors, remote-data fetch failures).
+3. CDN script load failure (network unavailable, blocked by CSP, jsdelivr outage, airgapped deployment).
+
+Error box markup (sketch — actual class names follow existing renderer conventions):
+
+```
+<div class="vega-error">
+  <strong>Vega render error</strong>
+  <p>{message}</p>
+  <pre>{original spec text}</pre>
+</div>
+```
+
+Styling: red 1 px border, padding for readability, monospace `<pre>`. Style lives alongside the helper or in `client/scss/`.
+
+For the dynamic-import failure case, the error is rendered on the first matched element and the failure is logged to `console.error`.
+
+---
+
+## Idempotency
+
+The editor preview re-renders markdown on each edit (debounced). Hydration must be safe to run repeatedly:
+
+- On first run for a given element, copy text content into `data-vega-spec` before calling `embed` (which replaces the element's children).
+- On subsequent runs, read from `data-vega-spec` so re-hydration doesn't see the rendered SVG/canvas as the source.
+- Each re-run replaces the rendered output via vega-embed's normal call.
+
+Because the editor preview replaces the entire preview pane's innerHTML on each render cycle, in practice the elements seen are fresh — the `data-vega-spec` mechanism is defensive against future changes to the preview pipeline.
+
+---
+
+## Testing
+
+**Server-side unit tests:**
+
+Add tests under `server/test/` (mirror existing renderer test patterns if any are present) confirming each renderer's `init` transforms `<pre class="prismjs"><code class="language-vega">…</code></pre>` into `<div class="vega">…</div>`, and the same for `vega-lite`.
+
+**Manual QA checklist:**
+
+- Live page view: render a Vega-Lite bar chart, render a full Vega scatter, both light and dark mode.
+- Editor preview: same charts, confirm hydration after edit + debounce.
+- Induced error case: malformed JSON, malformed spec, unreachable remote data URL — confirm error box renders.
+- Network tab: confirm CDN scripts do not load on a page without vega blocks; load exactly once on a page with one or more blocks.
+- Offline test: simulate CDN unreachable (devtools "Offline" or block jsdelivr.net) and confirm error box appears with "Failed to load Vega library" message.
+
+---
+
+## Out of Scope
+
+- Live theme reactivity (re-rendering on dark/light toggle without reload). Matches Mermaid behavior.
+- Custom or user-supplied Vega themes beyond vega-embed's built-ins.
+- Spec sandboxing (remote-data restrictions, expression-parser disabling). Trusted-editor model accepted.
+- Vega action menu / tooltip customization beyond `actions: false`.
+- Server-side pre-rendering. `vega-embed` is browser-only; SSR is not pursued.
+
+---
+
+## File Inventory
+
+New:
+- `server/modules/rendering/html-vega/definition.yml`
+- `server/modules/rendering/html-vega/renderer.js`
+- `server/modules/rendering/html-vega-lite/definition.yml`
+- `server/modules/rendering/html-vega-lite/renderer.js`
+- `client/components/vega/hydrate.js`
+- `client/scss/components/vega.scss`
+- `server/test/rendering/vega.test.js`
+
+Modified:
+- `client/themes/default/components/page.vue` — call hydrator in mounted hook.
+- `client/components/editor/editor-markdown.vue` — call hydrator in preview render path (mounted, processContent, previewShown watcher). The hydrator handles raw `<pre><code class="language-vega">` markup directly — no `markdown-it` `highlight()` modification needed.
+- `client/scss/app.scss` — import the new `components/vega` partial.
+
+Not modified:
+- `package.json` — no new dependencies (CDN strategy).
diff --git a/server/modules/rendering/html-vega-lite/definition.yml b/server/modules/rendering/html-vega-lite/definition.yml
new file mode 100644
index 00000000..27a332fd
--- /dev/null
+++ b/server/modules/rendering/html-vega-lite/definition.yml
@@ -0,0 +1,8 @@
+key: htmlVegaLite
+title: Vega-Lite
+description: Render Vega-Lite visualization specs from fenced code blocks
+author: couchbase-ps
+icon: mdi-chart-bar
+enabledDefault: true
+dependsOn: htmlCore
+props: {}
diff --git a/server/modules/rendering/html-vega-lite/renderer.js b/server/modules/rendering/html-vega-lite/renderer.js
new file mode 100644
index 00000000..2e24256f
--- /dev/null
+++ b/server/modules/rendering/html-vega-lite/renderer.js
@@ -0,0 +1,8 @@
+module.exports = {
+  init($, config) {
+    $('pre.prismjs > code.language-vega-lite').each((i, elm) => {
+      const vegaLiteContent = $(elm).html()
+      $(elm).parent().replaceWith(`<div class="vega-lite">${vegaLiteContent}</div>`)
+    })
+  }
+}
diff --git a/server/modules/rendering/html-vega/definition.yml b/server/modules/rendering/html-vega/definition.yml
new file mode 100644
index 00000000..7d5fcf89
--- /dev/null
+++ b/server/modules/rendering/html-vega/definition.yml
@@ -0,0 +1,8 @@
+key: htmlVega
+title: Vega
+description: Render Vega visualization specs from fenced code blocks
+author: couchbase-ps
+icon: mdi-chart-scatter-plot
+enabledDefault: true
+dependsOn: htmlCore
+props: {}
diff --git a/server/modules/rendering/html-vega/renderer.js b/server/modules/rendering/html-vega/renderer.js
new file mode 100644
index 00000000..d7699670
--- /dev/null
+++ b/server/modules/rendering/html-vega/renderer.js
@@ -0,0 +1,8 @@
+module.exports = {
+  init($, config) {
+    $('pre.prismjs > code.language-vega').each((i, elm) => {
+      const vegaContent = $(elm).html()
+      $(elm).parent().replaceWith(`<div class="vega">${vegaContent}</div>`)
+    })
+  }
+}
diff --git a/server/test/rendering/vega.test.js b/server/test/rendering/vega.test.js
new file mode 100644
index 00000000..95bf8247
--- /dev/null
+++ b/server/test/rendering/vega.test.js
@@ -0,0 +1,56 @@
+const cheerio = require('cheerio')
+
+describe('html-vega renderer', () => {
+  const renderer = require('../../modules/rendering/html-vega/renderer')
+
+  test('rewrites a fenced vega code block to a div.vega container', () => {
+    const html = `<pre class="prismjs"><code class="language-vega">{"$schema":"https://vega.github.io/schema/vega/v5.json"}</code></pre>`
+    const $ = cheerio.load(html)
+    renderer.init($, {})
+    const out = $.html()
+    expect(out).toContain('<div class="vega">')
+    expect(out).not.toContain('language-vega')
+    expect(out).toContain('vega.github.io/schema/vega/v5.json')
+  })
+
+  test('does not touch unrelated code blocks', () => {
+    const html = `<pre class="prismjs"><code class="language-js">console.log(1)</code></pre>`
+    const $ = cheerio.load(html)
+    renderer.init($, {})
+    const out = $.html()
+    expect(out).toContain('language-js')
+    expect(out).not.toContain('class="vega"')
+  })
+
+  test('does not match vega-lite blocks', () => {
+    const html = `<pre class="prismjs"><code class="language-vega-lite">{}</code></pre>`
+    const $ = cheerio.load(html)
+    renderer.init($, {})
+    const out = $.html()
+    expect(out).toContain('language-vega-lite')
+    expect(out).not.toContain('<div class="vega">')
+  })
+})
+
+describe('html-vega-lite renderer', () => {
+  const renderer = require('../../modules/rendering/html-vega-lite/renderer')
+
+  test('rewrites a fenced vega-lite code block to a div.vega-lite container', () => {
+    const html = `<pre class="prismjs"><code class="language-vega-lite">{"$schema":"https://vega.github.io/schema/vega-lite/v5.json"}</code></pre>`
+    const $ = cheerio.load(html)
+    renderer.init($, {})
+    const out = $.html()
+    expect(out).toContain('<div class="vega-lite">')
+    expect(out).not.toContain('language-vega-lite')
+    expect(out).toContain('vega.github.io/schema/vega-lite/v5.json')
+  })
+
+  test('does not match vega blocks', () => {
+    const html = `<pre class="prismjs"><code class="language-vega">{}</code></pre>`
+    const $ = cheerio.load(html)
+    renderer.init($, {})
+    const out = $.html()
+    expect(out).toContain('language-vega')
+    expect(out).not.toContain('<div class="vega-lite">')
+  })
+})