Omit empty srcset on content collection Markdown images

[jx-grxf]

Fixes #17871

Changes

• Markdown images rendered through a content collection always got a srcset attribute, so an image with no responsive candidates produced srcset="", which is invalid HTML and is flagged by the W3C validator.
• packages/astro/src/content/runtime.ts now only adds the attribute when image.srcSet.values.length > 0, which is the same guard packages/astro/src/vite-plugin-markdown/images.ts already applies. The <Image /> component behaves this way too.
• The serializer's value === '' branch is left alone, since it is what keeps an empty alt from being dropped.
• Added a changeset.

Testing

Added content collection images omit srcset when there are no candidates to packages/astro/test/core-image.test.ts. It reuses the existing core-image-ssg empty-alt fixture and the same selector as the neighbouring empty-alt test, so the two assertions cover both directions of the serializer's empty-string handling.

Verified against a standalone project on astro@7.2.9 (https://github.com/jx-grxf/astro-markdown-srcset-repro):

• default config, before: <img ... src="/api/public/proxy/github.com/_astro/wide...webp" srcset="">, after: no srcset
• image: { layout: 'constrained' }, after: srcset="/api/public/proxy/github.com/_astro/wide...webp 640w, ... 2000w" — still emitted when there are candidates
• the same Markdown in src/pages/legacy.md was already rendering without the attribute, before and after

Docs

No docs change. This aligns the content layer output with the documented behaviour of the other image paths; nothing user-facing gains or loses an option.

[changeset-bot]

🦋 Changeset detected

Latest commit: 3b945b7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 423 packages

Name	Type
astro	Patch
@e2e/astro-linked-lib	Patch
@e2e/actions-blog	Patch
@e2e/actions-react-19	Patch
@e2e/astro-component	Patch
@e2e/astro-envs	Patch
@e2e/astro-island-hydration-error	Patch
@test/astro-cloudflare-node-prerender-mdx	Patch
@test/astro-cloudflare	Patch
@e2e/content-collections	Patch
@e2e/csp-server-islands	Patch
@e2e/css	Patch
@test/custom-client-directives	Patch
@e2e/dev-toolbar	Patch
@e2e/error-cyclic	Patch
@e2e/error-sass	Patch
@e2e/errors	Patch
@e2e/hydration-race	Patch
@e2e/i18n	Patch
@test/nested-style-bug-e22e	Patch
@e2e/preact-compat-component	Patch
@e2e/preact-component	Patch
@e2e/preact-lazy-component	Patch
@e2e/prefetch	Patch
@e2e/react-component	Patch
@e2e/server-islands-key	Patch
@e2e/server-islands	Patch
@e2e/solid-circular	Patch
@e2e/solid-component	Patch
@e2e/solid-recurse	Patch
@e2e/svelte-component	Patch
@e2e/e2e-tailwindcss	Patch
@e2e/ts-resolution	Patch
@e2e/view-transitions	Patch
@e2e/vue-component	Patch
@performance/md	Patch
@performance/mdoc	Patch
@performance/mdx	Patch
@test/0-css	Patch
fake-astro-library	Patch
@test/actions	Patch
@test/alias-css-url-data-uri	Patch
@test/alias-css-url	Patch
@test/alias-path-alias-style	Patch
@test/ts-paths-no-baseurl	Patch
@test/aliases-tsconfig	Patch
@test/aliases	Patch
@test/api-routes	Patch
@test/asset-query-params-chunks	Patch
@test/asset-url-base	Patch
@test/astro-pages	Patch
@test/astro-assets-prefix	Patch
@test/astro-assets	Patch
@test/astro-basic	Patch
@test/astro-check-errors	Patch
@test/astro-check-no-errors	Patch
@test/astro-check-watch	Patch
@test/astro-children	Patch
@test/astro-client-only	Patch
@test/astro-component-bundling	Patch
@test/astro-component-code	Patch
@test/astro-css-bundling	Patch
@test/astro-dev-headers	Patch
@test/astro-dev-http2	Patch
@test/astro-doctype	Patch
@test/astro-dynamic	Patch
@test/astro-env-content-collections	Patch
@test/astro-env-required-public	Patch
@test/astro-env-server-fail	Patch
@test/astro-env-server-secret	Patch
@test/astro-env	Patch
@test/astro-envs	Patch
@test/astro-expr	Patch
@test/astro-get-static-paths	Patch
@test/astro-head	Patch
@test/astro-manifest-client-script	Patch
@test/astro-manifest-invalid	Patch
@test/astro-manifest	Patch
@test/astro-markdown-frontmatter-injection	Patch
@test/astro-markdown-plugins	Patch
@test/astro-markdown-remarkRehype	Patch
@test/astro-markdown-skiki-default-color	Patch
@test/astro-markdown-skiki-langs	Patch
@test/astro-markdown-skiki-themes-custom	Patch
@test/astro-markdown-skiki-themes-integrated	Patch
@test/astro-markdown-skiki-wrap-false	Patch
@test/astro-markdown-skiki-wrap-null	Patch
@test/astro-markdown-skiki-wrap-true	Patch
@test/astro-markdown-url	Patch
@test/astro-markdown	Patch
@test/astro-mode	Patch
@test/astro-page-directory-url	Patch
@test/astro-partial-html	Patch
@test/astro-preview-allowed-hosts	Patch
@test/astro-preview-headers	Patch
@test/astro-public	Patch
@test/astro-script-template-dedup	Patch
@test/astro-scripts	Patch
@test/astro-slots-nested	Patch
@test/concurrency	Patch
@test/build-readonly-file	Patch
@test/cache-memory-query-include	Patch
@test/cache-memory-query	Patch
@test/client-address-node	Patch
@test/client-only-child-styles	Patch
@test/client-only-css-chunk-leak	Patch
@test/code-component	Patch
@test/component-library	Patch
@test/config-vite-css-target-no-minify	Patch
@test/config-vite-css-target	Patch
@test/config-vite	Patch
@test/react-container	Patch
@test/content-with-spaces-in-folder-name	Patch
@test/content-collection-picture-render	Patch
@example/content-collection-references	Patch
@test/content-collection-tla-svg	Patch
@test/content-collections-base	Patch
@test/content-collections-empty-dir	Patch
@test/content-collections-empty-md-file	Patch
@test/content-collections-image-hmr	Patch
@test/content-collections-mutation	Patch
@test/content-collections-number-id	Patch
@test/content-collections-type-inference	Patch
@test/content-collections-with-config-mjs	Patch
@test/content-collections	Patch
@test/content-frontmatter	Patch
@test/content-intellisense	Patch
@test/content-layer-loader-schema-function	Patch
@test/content-layer-remark-plugins	Patch
@test/content-layer	Patch
@test/content-ssr-integration	Patch
@test/content-static-paths-integration	Patch
@test/content	Patch
@test/core-image-data-url	Patch
@test/core-image-deletion-ssr	Patch
@test/core-image-deletion	Patch
@test/core-image-errors	Patch
@test/core-image-fs-config	Patch
@test/core-image-remark-infersize	Patch
@test/core-image-layout	Patch
@test/core-image-picture-emit-file	Patch
@test/core-image-remark-imgattr	Patch
@test/core-image-ssg	Patch
@test/core-image-ssr	Patch
@test/core-image-svg-in-client	Patch
@test/core-image-svg	Patch
@test/core-image-unconventional-settings	Patch
@test/core-image	Patch
@test/csp-adapter	Patch
@test/csp-fonts	Patch
@test/csp	Patch
@test/css-assets	Patch
@test/css-dangling-references	Patch
@test/css-deduplication	Patch
@test/css-double-bundle	Patch
@test/css-dynamic-import-dev	Patch
@test/css-import-as-inline	Patch
@test/css-inline-stylesheets	Patch
@test/css-no-code-split	Patch
@test/css-path-case	Patch
@test/css-pure-chunk-query-params	Patch
@test/css-server-output-dedup	Patch
@test/custom-404-injected-from-dep	Patch
@test/custom-404-pkg	Patch
custom-fetch-error-pages	Patch
@test/custom-renderer	Patch
@test/data-collections-schema	Patch
@test/data-collections	Patch
@test/debug-component	Patch
@test/dev-container	Patch
@test/dev-render	Patch
@test/dev-request-url	Patch
@test/dynamic-endpoint-collision	Patch
@test/dynamic-route-build-file	Patch
@test/endpoint-routing	Patch
@test/error-bad-js	Patch
@test/error-build-location	Patch
@test/error-non-error	Patch
@test/extension-matching	Patch
@test/fetch	Patch
@test/fonts	Patch
@test/astro-fontsource-package	Patch
@test/get-static-paths-pages	Patch
@test/glob-pages-css	Patch
@test/head-propagation-prerender-env	Patch
@test/hmr-markdown	Patch
@test/hmr-middleware	Patch
@test/hmr-new-page	Patch
@test/hmr-slots-render	Patch
@test/hoisted-imports	Patch
@test/html-component	Patch
@test/html-escape	Patch
@test/html-page	Patch
@test/html-slots	Patch
@test/hydration-race	Patch
@test/i18n-client-import	Patch
@test/i18n-css-leak-basic	Patch
@test/import-ts-with-js	Patch
@test/impostor-md-file	Patch
@test/incremental-build-client-only	Patch
@test/incremental-build-concurrency	Patch
@test/incremental-build-content	Patch
@test/incremental-build-drop-key	Patch
@test/incremental-build-dynamic-import	Patch
@test/incremental-build-fonts	Patch
@test/incremental-build-headers	Patch
@test/incremental-build-images	Patch
@test/incremental-build-islands	Patch
@test/incremental-build-no-output	Patch
@test/incremental-build-script	Patch
@test/incremental-build	Patch
@test/integration-add-page-extension	Patch
@test/integration-server-setup	Patch
@test/jsx-queue-rendering	Patch
@test/large-array-solid	Patch
@test/legacy-collections-backwards-compat	Patch
@test/lightningcss-css-modules-content	Patch
@test/lightningcss-scoped-nesting	Patch
@test/live-loaders	Patch
@test/markdown	Patch
@test/middleware-dev	Patch
@test/middleware-full-ssr	Patch
@test/middleware-no-user-middlewaqre	Patch
@test/middleware-tailwind	Patch
@test/minification-html-default	Patch
@test/minification-html-jsx	Patch
@test/minification-html	Patch
@test/non-ascii-path	Patch
@test/non-html-pages	Patch
@test/page-format	Patch
@test/page-level-styles	Patch
@test/parallel-components	Patch
@test/partials-css-boundary	Patch
@test/partials	Patch
@test/passthrough-image-service	Patch
@test/postcss	Patch
@test/preact-compat-component	Patch
@test/preact-component	Patch
@test/remote-css	Patch
@test/request-signal	Patch
@test/reuse-injected-entrypoint	Patch
@test/root-srcdir-css	Patch
@test/scoped-style-strategy	Patch
@test/server-entry-fake-adapter	Patch
@test/server-entry	Patch
@test/server-islands-hybrid	Patch
@test/server-islands-ssr	Patch
@test/session-false	Patch
@test/session-tree-shake	Patch
@test/sessions	Patch
@test/slots-preact	Patch
@test/slots-react	Patch
@test/slots-solid	Patch
@test/slots-svelte	Patch
@test/slots-vue	Patch
@test/solid-component	Patch
@test/sourcemap	Patch
@test/space-in-folder-name	Patch
@test/special-chars-in-component-imports	Patch
@test/ssr-assets	Patch
@test/ssr-dynamic	Patch
@test/ssr-partytown	Patch
@test/ssr-prerender-get-static-paths	Patch
@test/ssr-prerender	Patch
@test/ssr-preview	Patch
@test/ssr-renderers-static-vue	Patch
@test/ssr-request	Patch
@test/ssr-hoisted-script	Patch
@test/ssr-scripts	Patch
@test/static-build-code-component	Patch
@test/static-build-dir	Patch
@test/static-build-frameworks	Patch
@test/static-build-page-url-format	Patch
@test/static-build-ssr	Patch
@test/static-build	Patch
@test/static-redirect	Patch
@test/svelte-component	Patch
@test/svg-deduplication	Patch
@test/tailwindcss	Patch
@e2e/third-party-astro	Patch
@test/url-import-suffix	Patch
@test/view-transitions	Patch
@test/virtual-astro-file	Patch
@test/vite-virtual-modules	Patch
@test/vitest	Patch
@test/vue-component	Patch
@test/vue-with-multi-renderer	Patch
@test/alpinejs-basics	Patch
@test/alpinejs-directive	Patch
@test/alpinejs-plugin-script-import	Patch
@test/astro-cloudflare-allowed-hosts	Patch
@test/astro-cloudflare-astro-dev-platform	Patch
@test/astro-cloudflare-astro-env	Patch
@test/astro-cloudflare-binding-image-cache	Patch
@test/astro-cloudflare-binding-image-service	Patch
@test/astro-cloudflare-cache-provider-wait-until	Patch
@test/astro-cloudflare-cache-provider	Patch
@test/astro-cloudflare-client-address	Patch
@test/astro-cloudflare-compile-custom-image-service	Patch
@test/astro-cloudflare-compile-image-service	Patch
@test/astro-cloudflare-custom-entryfile-fetch-state	Patch
@test/astro-cloudflare-custom-entryfile	Patch
@test/astro-cloudflare-custom-image-service	Patch
@test/astro-cloudflare-dev-image-endpoint	Patch
@test/astro-cloudflare-external-image-service	Patch
@test/astro-cloudflare-external-redirects	Patch
@test/cloudflare-incremental-images	Patch
@test/astro-cloudflare-internal-redirects	Patch
@test/astro-cloudflare-no-output	Patch
@test/astro-cloudflare-prerender-node-env	Patch
@test/astro-cloudflare-prerender-queue-consumers	Patch
@test/astro-cloudflare-prerender-styles	Patch
@test/astro-cloudflare-prerenderer-errors	Patch
@test/astro-cloudflare-prerenderer-render-error	Patch
@test/routing-priority-cloudflare	Patch
@test/cf-server-entry	Patch
@test/astro-cloudflare-server-island-prerender-framework	Patch
@test/astro-cloudflare-sql-import	Patch
@test/cf-ssr-deps	Patch
@test/astro-cloudflare-static	Patch
@test/astro-cloudflare-svelte-rune-deps	Patch
@test/astro-cloudflare-top-level-return	Patch
@test/cf-user-optimize-deps	Patch
@test/astro-cloudflare-vite-plugin	Patch
@test/astro-cloudflare-with-base	Patch
@test/astro-cloudflare-with-react	Patch
@test/astro-cloudflare-with-solid-js	Patch
@test/astro-cloudflare-with-svelte	Patch
@test/astro-cloudflare-with-vue	Patch
@test/astro-cloudflare-wrangler-preview-platform	Patch
@test/markdoc-content-collections	Patch
@test/content-layer-markdoc	Patch
@test/headings-custom	Patch
@test/headings	Patch
@test/image-assets-custom	Patch
@test/image-assets	Patch
@test/markdoc-propagated-assets	Patch
@test/markdoc-render-with-space	Patch
@test/markdoc-render-html	Patch
@test/markdoc-render-null	Patch
@test/markdoc-render-partials	Patch
@test/markdoc-render-simple	Patch
@test/markdoc-render-table-attrs	Patch
@test/markdoc-render-this-context	Patch
@test/markdoc-render-typographer	Patch
@test/markdoc-render-with-components	Patch
@test/markdoc-render-with-config	Patch
@test/markdoc-render-with-extends-components	Patch
@test/markdoc-render-with-indented-components	Patch
@test/markdoc-render-with-transform	Patch
@test/markdoc-variables	Patch
@test/content-layer-rendering	Patch
@test/mdx-css-head-mdx	Patch
@test/image-remark-imgattr	Patch
@test/mdx-astro-container-escape	Patch
@test/mdx-frontmatter-injection	Patch
@test/netlify-skew-protection	Patch
@test/netlify-hosted-astro-project	Patch
@test/nodejs-api-route	Patch
@test/nodejs-badurls	Patch
@test/nodejs-encoded	Patch
@test/nodejs-errors	Patch
@test/nodejs-headers	Patch
@test/nodejs-image	Patch
@test/locals	Patch
@test/node-middleware	Patch
@test/nodejs-prerender-404-500	Patch
@test/nodejs-prerender	Patch
@test/nodejs-prerendered-error-page-fetch	Patch
@test/nodejs-preview-headers	Patch
@test/redirects	Patch
@test/node-sessions	Patch
@test/ssr-assets-middleware	Patch
@test/node-static-headers	Patch
@test/node-trailingslash	Patch
@test/url	Patch
@test/well-known-locations	Patch
@test/react-component	Patch
@test/sitemap-chunks	Patch
@test/sitemap-dynamic	Patch
@test/sitemap-i18n-fallback	Patch
@test/sitemap-ssr	Patch
@test/sitemap-static	Patch
@test/sitemap-trailing-slash	Patch
async-rendering	Patch
conditional-rendering	Patch
@test/empty-class	Patch
svelte-prop-types	Patch
@test/astro-vercel-basic	Patch
@test/astro-vercel-image	Patch
@test/astro-vercel-integration-assets	Patch
@test/vercel-isr-edge-no-middleware	Patch
@test/vercel-isr-with-edge-middleware	Patch
@test/vercel-isr	Patch
@test/vercel-max-duration	Patch
@test/vercel-edge-middleware-with-edge-file	Patch
@test/vercel-edge-middleware-without-edge-file	Patch
@test/astro-vercel-no-output	Patch
@test/astro-vercel-outdir-outside-root	Patch
@test/astro-vercel-prerendered-error-pages	Patch
@test/astro-vercel-redirects-serverless	Patch
@test/astro-vercel-redirects	Patch
@test/vercel-server-islands-static	Patch
@test/vercel-server-islands	Patch
@test/astro-vercel-serverless-prerender	Patch
@test/astro-vercel-serverless-with-dynamic-routes	Patch
@test/astro-vercel-static-assets	Patch
@test/vercel-static-headers	Patch
@test/astro-vercel-static	Patch
@test/vercel-streaming	Patch
@test/astro-vercel-with-web-analytics-enabled-output-as-static	Patch
vercel-hosted-astro-project	Patch
@test/vue-app-entrypoint-async	Patch
@test/vue-app-entrypoint-css	Patch
@test/vue-app-entrypoint-no-export-default	Patch
@test/vue-app-entrypoint-relative	Patch
@test/vue-app-entrypoint-src-absolute	Patch
@test/vue-app-entrypoint	Patch
@test/vue-basics	Patch
vue-prop-types	Patch
astro-benchmark	Patch
@benchmark/adapter	Patch
@benchmark/timer	Patch

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[codspeed-hq]

Merging this PR will degrade performance by 14.04%

❌ 1 (👁 1) regressed benchmark
✅ 20 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
👁	Simulation	Build: hybrid site (static + server)	1.5 s	1.7 s	-14.04%

---

_{Comparing jx-grxf:fix/content-image-empty-srcset (3b945b7) with main (2548abf)}

[jx-grxf]

The CodSpeed regression looks like noise rather than a real effect of this change.

The only benchmark that moved is Build: hybrid site (static + server), and its fixture in benchmark/static-projects/build-hybrid/ contains no Markdown files and no content collections — every route is .astro, plus one JSON endpoint:

src/pages/index.astro
src/pages/about.astro
src/pages/dashboard.astro
src/pages/blog/index.astro
src/pages/blog/[id].astro
src/pages/api/data.json.js

The code this PR touches is inside updateImageReferences in packages/astro/src/content/runtime.ts, which only runs while rendering a content-collection entry's HTML. That benchmark never reaches it, so it cannot be executing the changed line at all.

The change itself is a conditional spread on an array length that replaces an unconditional property assignment, on a path that runs once per Markdown image. The benchmark runs 3 iterations of a full build measured in wall-clock time, so a 1.5s → 1.7s swing on a shared runner is well within run-to-run variance.

Happy to re-run the benchmark job if that would help confirm it.