Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesPut the complete data URL in an image element’s src attribute:
<img src="data:image/png;base64,BASE64_IMAGE_BYTES" alt="Description of the image">Use data:image/...;base64, for binary image bytes, or percent-encode text such as SVG. Keep the payload small, provide useful alt text, and check your Content Security Policy (CSP) if the browser refuses to load it.
The HTML syntax
A data URL follows this form:
data:[<mediatype>][;base64],<data>The comma separates metadata from the payload. For an image, declare the actual media type, then place the entire URL in src:
<img
src="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 1 1'%3E%3Crect width='1' height='1' fill='red'/%3E%3C/svg%3E"
alt="Red square"
width="32"
height="32"
>This example embeds SVG text without Base64. The HTML img element is intended for one image resource; the representation used in src does not change normal accessibility requirements. The original syntax is defined by RFC 2397, while the living HTML Standard defines image-element behavior.
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Binary image example
For PNG, JPEG, GIF, WebP, or another binary format, use standard Base64:
<img
src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB..."
alt="One-pixel transparent image"
width="1"
height="1"
>The shortened bytes above are illustrative only; a browser needs the complete Base64 representation. Match the media type to the bytes. For example, JPEG bytes require image/jpeg, not image/png.
Base64 or percent-encoding?
Use Base64 for binary bytes
Base64 converts arbitrary bytes into text that can travel inside an HTML attribute. A typical value is:
data:image/webp;base64,UklGR...- Keep the
;base64,marker; without it, the browser interprets the payload as non-Base64 data. - Use standard Base64, including its normal padding characters when present. URL-safe Base64 substitutions are not automatically interchangeable.
- Do not insert line breaks or surrounding quotation marks into the payload.
Use percent-encoding for textual SVG
SVG is text, so it can be embedded without ;base64 when characters that are unsafe in a URL are escaped:
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data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='80' height='40'%3E%3Ctext x='4' y='25'%3EHello%3C/text%3E%3C/svg%3ESpaces, line breaks, percent signs, quotes, angle brackets, and other reserved or non-printing characters may need percent-encoding. The exact payload is opaque to URL processing; encode it deliberately rather than pasting unescaped markup into a complex attribute.
Why the comma matters
Everything before the first comma describes the media type and optional encoding. Everything after it is the data. A missing comma, a second accidental prefix, or a copied value such as data:image/png;base64,data:image/png... makes the resource invalid.
Creating data URLs safely
Browser-side JavaScript for a selected file
This small page reads a local file and displays it as a data URL. It does not upload the file:
<input id="file" type="file" accept="image/*">
<img id="preview" alt="Selected image preview">
<script>
const fileInput = document.querySelector('#file');
const preview = document.querySelector('#preview');
fileInput.addEventListener('change', () => {
const file = fileInput.files[0];
if (!file) return;
if (!file.type.startsWith('image/')) {
preview.removeAttribute('src');
return;
}
const reader = new FileReader();
reader.addEventListener('load', () => {
preview.src = reader.result;
});
reader.readAsDataURL(file);
});
</script>For production interfaces, consider rejecting unexpectedly large files before reading them and preserving the original MIME type supplied by the file object only after validating the content your application accepts.
Python: convert a local image
from pathlib import Path
import base64
path = Path("logo.png")
mime = "image/png"
payload = base64.b64encode(path.read_bytes()).decode("ascii")
data_url = f"data:{mime};base64,{payload}"
print(data_url)Node.js: convert a local image
import { readFileSync } from "node:fs";
const bytes = readFileSync("logo.png");
const dataUrl = `data:image/png;base64,${bytes.toString("base64")}`;
console.log(dataUrl);Shell/cURL pipeline
On systems with the standard base64 utility, this prints a PNG data URL. The exact wrapping options differ between platforms, so ensure the output has no line breaks:
printf 'data:image/png;base64,'
printf 'logo.png' | base64 | tr -d 'n'
printf 'n'cURL itself transports bytes; it does not decide the correct media type or create a data URL automatically. Validate both pieces before inserting the result into HTML.
Accessibility and valid HTML
Write replacement text in alt
Describe the image’s purpose, not its encoding. For an account avatar, use a person’s name; for a chart, summarize the useful result. A data URL does not make an image exempt from accessibility rules.
<img src="data:image/svg+xml,..." alt="Green check mark">Use an empty value for decorative art
If nearby text already conveys the image’s meaning or the image is purely decorative, use alt="". Do not omit the attribute: omission can cause assistive technology to announce the file or URL instead of treating it as decorative.
Reserve dimensions when layout stability matters
Adding width and height (or an equivalent aspect-ratio rule) lets the browser reserve space before decoding the embedded bytes. This is especially useful when several inline images appear together.
When a data URL is the wrong choice
| Situation | Prefer | Reason |
|---|---|---|
| One tiny icon used once | Data URL | No separate request is needed and the markup stays manageable. |
| Large photograph, hero image, or screenshot | Normal image URL | Base64 increases the textual size and makes the HTML document heavier. |
| Same image appears on many pages | Normal image URL | A separate resource can be cached and reused without duplicating bytes in every document. |
| Image changes independently of page markup | Normal image URL | Updating a file is simpler than regenerating many documents. |
Strict CSP that excludes data: |
Hosted image or an explicitly reviewed policy change | The browser will enforce the policy even when the data URL is syntactically correct. |
RFC 2397 describes data URLs as useful for short values. Base64 is also larger than the original binary representation, and browser maximum URL lengths vary. There is no portable, browser-wide “maximum image data URL” number to design around; test the actual browsers and document sizes you support.
Fixing a blocked or broken image
1. Inspect the console and response policy
A CSP violation normally appears in the browser developer console. The img-src directive controls permitted image and favicon sources. If img-src is absent, the browser falls back to default-src. A policy such as:
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Content-Security-Policy: default-src 'self'does not permit a data URL for images. If your security review allows it, add the scheme only to the image directive:
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Content-Security-Policy: default-src 'self'; img-src 'self' data:Do not add data: broadly to script-src, object-src, or unrelated directives. Follow the site’s security policy and test the change in a report-only policy first when appropriate. A CSP reference is available from MDN’s Content Security Policy documentation.
2. Check the prefix and payload
- Confirm the value begins with
data:and contains exactly one metadata/payload comma. - Use a real image MIME type such as
image/png,image/jpeg,image/gif,image/webp, orimage/svg+xml. - For binary bytes, confirm
;base64,is present and that the Base64 string was not URL-decoded, truncated, or wrapped with whitespace. - For SVG text, percent-encode markup characters and ensure the XML is well formed.
3. Check the HTML parser context
Use quoted attributes when the URL contains punctuation. Server-side templating, JSON serialization, or a sanitizer may escape, truncate, or remove the value. Inspect the final DOM in developer tools rather than only the source template.
4. Check the image bytes
Download or decode the payload independently and open the resulting file. A correct prefix cannot repair corrupted bytes or a file whose content is not the declared type. An img element must not load a non-image resource as an image.
Security and URL behavior
Data URLs are not a way to bypass security controls. Modern browsers treat navigated data URLs as unique opaque origins, and top-level navigation to them is restricted as a security mitigation. Do not put untrusted, unsanitized user input into an SVG data URL: although an image resource is not supposed to execute embedded code as script, unsafe markup can still create parsing, spoofing, or denial-of-service problems in surrounding systems. Sanitize untrusted content and enforce CSP.
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A data URL has no relative-URL form. Appending ?version=2 does not create a normal query string for cache-busting; the data portion is opaque. If you need independent cache keys, access control, or content negotiation, use a normal URL served by your application or a CDN.
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Practical checklist
- Start with
<img src="data:..." alt="...">. - Declare the actual image media type.
- Use Base64 for binary bytes and percent-encoding for textual SVG.
- Keep the payload short and avoid duplicating large or frequently reused images.
- Give informative
alttext, oralt=""for decorative art. - Check
img-srcand itsdefault-srcfallback when CSP blocks the image. - Inspect the final DOM and independently decode suspicious payloads.
Frequently Asked Questions
Can I use a data URL in an image’s srcset?
Yes, but each candidate must be a complete, correctly escaped URL and the commas used by srcset descriptors make maintenance difficult. A normal image URL is usually clearer for responsive images.
Does omitting the media type make an image work?
The data URL default is text/plain;charset=US-ASCII, not an image type. Always declare the image media type explicitly.
Can I add a cache-busting query string to a data URL?
No. A data URL has no relative URL form, and its payload is opaque; use a separate resource URL when you need normal query parameters or cache control.
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