Data URI scheme

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The data URI scheme is a URI scheme (Uniform Resource Identifier scheme) that provides a way to include data in-line in web pages as if they were external resources. It is a form of file literal or here document. This technique allows normally separate elements such as images and style sheets to be fetched in a single HTTP request rather than multiple HTTP requests, which can be more efficient.

Data URIs tends to be simpler than other inclusion methods, such as MIME with cid or mid URIs. Data URIs are sometimes called Uniform Resource Locators, although they do not actually locate anything remote. The data URI scheme is defined in RFC 2397 of the Internet Engineering Task Force (IETF).

In browsers that fully support Data URIs for "navigation", JavaScript-generated content can be provided as file "download" to the user, simply by setting window.location.href to a Data URI. One example is the conversion of HTML tables to downloadable CSV using a Data URI like this: 'data:text/csv;charset=UTF-8,' + encodeURIComponent(csv), where "csv" has been generated by JavaScript.

The IETF published the data URI specification in 1998[1] as Proposed Standard on the IETF Standards Track, and hasn't progressed it since. The HTML 4.01 specification refers to the data URI scheme,[2] and data URIs have now been implemented in most browsers.

Web browser support[edit]

As of March 2012, Data URIs are supported by the following web browsers:

  • Gecko-based, such as Firefox, SeaMonkey, XeroBank, Camino, Fennec and K-Meleon
  • Konqueror, via KDE's KIO slaves input/output system
  • Opera (including devices such as the Nintendo DSi or Wii)
  • WebKit-based, such as Safari (including iOS), Android's browser, Kindle 4's browser, Epiphany and Midori (WebKit is a derivative of Konqueror's KHTML engine, but Mac OS X does not share the KIO architecture so the implementations are different), and Webkit/Chromium-based, such as Chrome
  • Trident
    • Internet Explorer 8: Microsoft has limited its support to certain "non-navigable" content for security reasons, including concerns that JavaScript embedded in a data URI may not be interpretable by script filters such as those used by web-based email clients. Data URIs must be smaller than 32 KB in Version 8.[3] Data URIs are supported only for the following elements and/or attributes:[4]
      • object (images only)
      • img
      • input type=image
      • link
      • CSS declarations that accept a URL, such as background-image, background, list-style-type, list-style and similar.
    • Internet Explorer 9: Internet Explorer 9 does not have 32KB limitation and supports more elements.

Email Client support[edit]

Following clients support data URI for images[3]

Format[edit]

data:[<MIME-type>][;charset=<encoding>][;base64],<data>

The encoding is indicated by ;base64. If it is present the data is encoded as base64. Without it the data (as a sequence of octets) is represented using ASCII encoding for octets inside the range of safe URL characters[5] and using the standard %xx hex encoding of URLs for octets outside that range. If <MIME-type> is omitted, it defaults to text/plain;charset=US-ASCII. (As a shorthand, the type can be omitted but the charset parameter supplied.)

Some browsers (Chrome, Opera, Safari, Firefox) accept a non-standard ordering if both ;base64 and ;charset are supplied, while Internet Explorer requires that the charset's specification must precede the base64 token.

Advantages and disadvantages[edit]

Advantages[edit]

  • HTTP request and header traffic is not required for embedded data, so data URIs consume less bandwidth whenever the overhead of encoding the inline content as a data URI is smaller than the HTTP overhead. For example, the required base64 encoding for an image 600 bytes long would be 800 bytes, so if an HTTP request required more than 200 bytes of overhead, the data URI would be more efficient.
  • For transferring many small files (less than a few kilobytes each), this can be faster. TCP transfers tend to start slowly. If each file requires a new TCP connection, the transfer speed is limited by the round-trip time rather than the available bandwidth. Using HTTP keep-alive improves the situation, but may not entirely alleviate the bottleneck.
  • While web browsers will not cache inline-loaded data as separate resource, external CSS files using data URIs are cached, so that an external CSS file with 10 background-images embedded as data URIs requires only one initial request instead of eleven and subsequent requests require only retrieving one cached CSS file, instead of one CSS file plus ten cached images.
  • When browsing a secure HTTPS web site, web browsers commonly require that all elements of a web page be downloaded over secure connections, or the user will be notified of reduced security due to a mixture of secure and insecure elements. On badly configured servers, HTTPS requests have significant overhead over common HTTP requests, so embedding data in data URIs may improve speed in this case.
  • Web browsers are usually configured to make only a certain number of concurrent HTTP connections to a domain (although the IETF recommendation is 2, most current browsers use 6 or more)[6][7] so inline data frees up a download connection for other content.
  • Environments with limited or restricted access to external resources may embed content when it is disallowed or impractical to reference it externally. For example, an advanced HTML editing field could accept a pasted or inserted image and convert it to a data URI to hide the complexity of external resources from the user. Alternatively, a browser can convert (encode) image based data from the clipboard to a data URI and paste it in a HTML editing field. Mozilla Firefox 4 supports this functionality.
  • It is possible to manage a multimedia page as a single file.
  • Email message templates can contain images (for backgrounds or signatures) without the image appearing to be an "attachment".

Disadvantages[edit]

  • Data URIs are not separately cached from their containing documents (e.g. CSS or HTML files), therefore the encoded data is downloaded every time the containing documents are re-downloaded.
  • Content must be re-encoded and re-embedded every time a change is made.
  • Internet Explorer through version 7 (less than 1% of web traffic as of June 2014),[8] lacks support. However this can be overcome by serving browser-specific content.[9]
  • Internet Explorer 8 limits data URIs to a maximum length of 32 KB. (Internet Explorer 9 does not have this limitation)[3][4]
  • In IE 8 and 9, data URIs can only be used for images, but not for navigation or JavaScript generated file downloads.[10]
  • Data URIs are included as a simple stream, and many processing environments (such as web browsers) may not support using containers (such as multipart/alternative or message/rfc822) to provide greater complexity such as metadata, data compression, or content negotiation.
  • Base64-encoded data URIs are 1/3 larger in size than their binary equivalent. (However, this overhead is reduced to 2–3% if the HTTP server compresses the response using gzip)[11] (this holds for white noise data; in generally, real-world image data with standard web compression will result in overhead of around 10%).
  • Data URIs do not carry a file name as a normal linked file would. When saving, a browser may offer a random identifier or a default file name for the specified MIME. However, an HTML link with a reference to a data URI may provide a file name using the download attribute.
  • Referencing the same resource (such as an embedded small image) more than once from the same document results in multiple copies of the embedded resource. In comparison, an external resource can be referenced arbitrarily many times, yet downloaded and decoded only once.
  • Data URIs make it more difficult for security software to filter content.[12]

Examples[edit]

HTML[edit]

An HTML fragment embedding a picture of small red dot: Red-dot-5px.png

<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA
AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO
9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot" />

As demonstrated above, data URIs encoded with base64 may contain whitespace for readability.

CSS[edit]

A CSS rule that includes a background image:

ul.checklist li.complete {
    padding-left: 20px;
    background: white url('data:image/png;base64,iVBORw0KGgoAA
AANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEUAAAD///+l2Z/dAAAAM0l
EQVR4nGP4/5/h/1+G/58ZDrAz3D/McH8yw83NDDeNGe4Ug9C9zwz3gVLMDA/A6
P9/AFGGFyjOXZtQAAAAAElFTkSuQmCC') no-repeat scroll left top;
}

In Mozilla Firefox 5, Google Chrome 17, and IE 9 (released June, 2011), encoded data must not contain newlines.

JavaScript[edit]

A JavaScript statement that opens an embedded subwindow, as for a footnote link:

window.open('data:text/html;charset=utf-8,' + 
    encodeURIComponent( // Escape for URL formatting
        '<!DOCTYPE html>'+
        '<html lang="en">'+
        '<head><title>Embedded Window</title></head>'+
        '<body><h1>42</h1></body>'+
        '</html>'
    )
);

This example does not work with Internet Explorer 8 due to its security restrictions that prevent navigable file types from being used.[4]

See also[edit]

  • MHTML, an alternative for attaching resources to an HTML document usually found in HTML email messages
  • MIME for the used mediatypes

References[edit]

  1. ^ Masinter, L (August 1998). "RFC 2397 - The "data" URL scheme". Internet Engineering Task Force. Retrieved 2008-08-12. 
  2. ^ Raggett, Dave; Le Hors, Arnaud; Jacobs, Ian (1999-12-24). "Objects, Images, and Applets: Rules for rendering objects". HTML 4.01 Specification. World Wide Web Consortium. Retrieved 2008-03-20. 
  3. ^ a b c "IE9 Beta Minor Changes List". Eric Law. Retrieved 2010-10-28. 
  4. ^ a b c "data Protocol". Microsoft Developer Network. Microsoft. Retrieved 2009-01-05. 
  5. ^ "RFC2397 Errata 2045". IETF. Retrieved 5 July 2014. "dataurl := "data:" [ mediatype ] [ ";base64" ] "," data … data := *uric … where "uric" is imported from [RFC2396]" 
  6. ^ "RFC 2616 Section 8.1.4". Internet Engineering Task Force. 
  7. ^ Steve Souders. "Roundup on Parallel Connections". 
  8. ^ "StatCounter Global Stats". 
  9. ^ "CSS Inline Images and the Data URI". 
  10. ^ "Data URI support in Microsoft browsers". Retrieved 2012-04-09. 
  11. ^ Isenburg, Martin; Snoeyink, Jack (2003). Binary Compression Rates for ASCII Formats. CiteSeerX: 10.1.1.13.7233. 
  12. ^ Masinter, L (August 1998). "Security". RFC 2397 - The "data" URL scheme. Internet Engineering Task Force. p. 2. Retrieved 2008-08-12. 

External links[edit]