Self-hosting a font, image, or script works only when the browser can fetch the right file at the right URL and the response passes the site’s security rules. Check the deployed URL, response headers, origin, and Content Security Policy (CSP) for each asset; a file existing on your server is not enough.
Start by checking each asset request
Make an inventory of the font, image, and JavaScript URLs your page requests, then inspect them in the browser’s developer tools under Network. For each request, check its status, final URL after redirects, response headers, and whether it is same-origin or cross-origin. Use the console to spot CSP, CORS, MIME-type, or network errors.
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Keep paths consistent across your HTML, CSS, build output, and server routing. A path that works locally can fail after deployment if the base path, letter case, or deployment directory differs. Server-specific configuration depends on your hosting stack, so verify the actual deployed responses rather than assuming a local setup applies.
How to self-host fonts
Point @font-face at the deployed file
Declare the font with @font-face and set src to the font’s deployed URL. Ensure the declared family, weight, and style match the font file you intend the browser to use. If the font falls back to another typeface, inspect the URL and response first, then check the declaration and the site’s policy.
Check CSP and cross-origin access
CSP’s font-src directive controls permitted font sources; 'self' allows same-origin font files. If the file is hosted on another origin, that server may need to grant CORS access. Web fonts are among the resource types for which CORS can matter. A permitted CSP source does not replace the required CORS response from a cross-origin font host.
How to self-host images
Use the image’s deployed URL and inspect the response status if it does not appear. CSP’s img-src directive controls which sources may provide images, so check it when images disappear after a policy change.
Displaying an image and reading its pixels through a canvas are different cases. If your code draws a cross-origin image to a canvas and needs to read pixel data, cross-origin permissions can matter even when the image itself displays. Keep the allowed sources no broader than the behavior your page needs. See MDN’s CORS guide for the relevant cross-origin cases.
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Verify the file and its MIME type
Serve JavaScript with the standard JavaScript media type text/javascript. If the response has X-Content-Type-Options: nosniff, browsers block scripts served with an invalid JavaScript MIME type. A request can return a successful status yet still fail to run: inspect the response body and Content-Type to make sure the server returned JavaScript rather than an HTML fallback page, such as a not-found route. MDN documents the behavior in its MIME type guide and MIME verification reference.
Allow only the script sources the page needs
CSP’s script-src directive governs permitted script sources. If a script is blocked, identify the specific resource and whether the page needs it before changing the policy; widening the allowlist just to silence an error can allow more than intended. MDN recommends strict nonce- or hash-based policies where practical and testing policy changes in report-only mode before enforcing them. See its CSP implementation guide.
Keep a CDN script with Subresource Integrity
If you retain a third-party script, Subresource Integrity (SRI) lets the page require a known hash for the downloaded bytes. The hash must match the exact file, and the request should use HTTPS. For a cross-origin SRI request, the resource server must permit CORS and the markup must include crossorigin, commonly crossorigin="anonymous" for public resources that do not use credentials. SRI checks that fetched content matches the pinned hash; it does not make the pinned code safe if that code is malicious. See MDN’s SRI implementation guide and cross-origin SRI reference.
Use CSP directives that match each asset type
Fonts, images, and scripts have separate CSP controls: font-src, img-src, and script-src. Allowing an origin for one kind of asset does not automatically permit it for the others. When changing a policy, test it using Content-Security-Policy-Report-Only, review violations, and then allow only the sources the page actually requires. MDN’s font-src reference and CSP header reference describe these controls.
Preload only assets needed early
Preload a file only if the current page needs it early in rendering. MDN’s font preload example uses rel="preload", as="font", a font type, and crossorigin. Do not add speculative preloads for assets the page does not use: they consume resources without helping that page. For JavaScript modules, modulepreload hints that the browser should start downloading modules at higher priority. Measure the page’s actual request behavior before adding multiple speculative loads; a hint does not establish a universal performance gain. See MDN’s speculative loading guide.
Same-origin hosting or a CDN?
Choose based on the site’s delivery needs and the operational work each option creates. Same-origin hosting can simplify origin permissions and CSP configuration. Cross-origin delivery can require explicit CORS setup and policy maintenance. Consider control over deployment and caching as well, but do not assume either approach is universally faster, safer, or cheaper: those outcomes depend on the site and configuration and require measurements to establish.
Quick Recap
Troubleshoot by symptom
- Font falls back: Check the font URL, response, and
@font-facefamily, weight, and style; then inspect CSPfont-srcand, for a cross-origin font, CORS. - Image is missing: Check the deployed URL, response status, and CSP
img-src. If code reads canvas pixels, check the relevant cross-origin permissions too. - Script request succeeds but code does not run: Inspect the response body and
Content-Type, confirm it is JavaScript rather than an HTML fallback, and checknosniffand CSPscript-src. - Integrity-checked external script is blocked: Verify the hash against the exact file, use HTTPS, and confirm the server supports CORS and the markup includes
crossorigin. - A CSP change breaks assets: Test the policy in report-only mode, review the violations, and add only the sources the page needs.
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