Data URLs

Data URLs, URLs prefixed with the data: scheme, allow content creators to embed small files inline in documents.

Syntax

Data URLs are composed of four parts: a prefix (data:), a MIME type indicating the type of data, an optional base64 token if non-textual, and the data itself:

data:[<mediatype>][;base64],<data>

The mediatype is a MIME type string, such as 'image/jpeg' for a JPEG image file. If omitted, defaults to text/plain;charset=US-ASCII

If the data is textual, you can simply embed the text (using the appropriate entities or escapes based on the enclosing document's type). Otherwise, you can specify base64 to embed base64-encoded binary data.

A few examples:

data:,Hello%2C%20World!

Simple text/plain data

data:text/plain;base64,SGVsbG8sIFdvcmxkIQ%3D%3D

base64-encoded version of the above

data:text/html,%3Ch1%3EHello%2C%20World!%3C%2Fh1%3E

An HTML document with <h1>Hello, World!</h1>

data:text/html,<script>alert('hi');</script>

An HTML document that executes a JavaScript alert. Note that the closing script tag is required.

Encoding data into base64 format

This can be done easily using the command-line uuencode utility on Linux and Mac OS X systems:

uuencode -m infile remotename

The infile parameter is the name of the file you wish to encode into base64 format, and remotename is the remote name for the file, which isn't actually used in data URLs.

The output will look something like this:

begin-base64 664 test
YSBzbGlnaHRseSBsb25nZXIgdGVzdCBmb3IgdGV2ZXIK
====

The data URL will use the encoded data after the initial header line.

In a Web page, using JavaScript

The Web APIs has primitives to encode or decode to base64: Base64 encoding and decoding.

Common problems

This section describes problems that commonly occur when creating and using data URLs.

Syntax

The format for data URLs is very simple, but it's easy to forget to put a comma before the "data" segment, or to incorrectly encode the data into base64 format.

Formatting in HTML

A data URL provides a file within a file, which can potentially be very wide relative to the width of the enclosing document. As a URL, the data should be formatable with whitespace (linefeed, tab, or spaces), but there are practical issues that arise when using base64 encoding.

Length limitations

Although Firefox supports data URLs of essentially unlimited length, browsers are not required to support any particular maximum length of data. For example, the Opera 11 browser limited URLs to 65535 characters long which limits data URLs to 65529 characters (65529 characters being the length of the encoded data, not the source, if you use the plain data:, without specifying a MIME type).

Lack of error handling

Invalid parameters in media, or typos when specifying 'base64', are ignored, but no error is provided.

No support for query strings, etc.

The data portion of a data URL is opaque, so an attempt to use a query string (page-specific parameters, with the syntax <url>?parameter-data) with a data URL will just include the query string in the data the URL represents. For example:

data:text/html,lots of text...<p><a name%3D"bottom">bottom</a>?arg=val

This represents an HTML resource whose contents are:

lots of text...<p><a name="bottom">bottom</a>?arg=val

Specifications

Browser compatibility

1. The maximum size supported is 4GB

2. The maximum size supported is 32kB

See also

• Base64 encoding and decoding
• atob()
• btoa()
• CSS url()
• URI