Frequently Asked Questions

A Data URL is a special type of URL that allows you to embed small files directly into HTML, CSS, or JavaScript code as a string of text. Instead of pointing to a file location, a Data URL contains the actual content of the file, typically encoded in Base64 format.

Data URLs begin with the data: scheme identifier, followed by a MIME type, an optional Base64 indicator, and the encoded data itself.

There is no functional difference between a Data URL and a Data URI. The terms are used interchangeably in web development.

Technically, "URI" (Uniform Resource Identifier) is a broader term that encompasses both "URL" (Uniform Resource Locator) and "URN" (Uniform Resource Name). In practice, when referring to the data: scheme, both "Data URL" and "Data URI" refer to the same thing.

The general syntax of a Data URL is:

data:[<media type>][;base64],<data>

Where:

• data: is the scheme identifier
• <media type> is an optional MIME type (e.g., image/png, text/plain). If omitted, it defaults to text/plain;charset=US-ASCII
• ;base64 is an optional indicator that the data is Base64 encoded
• <data> is the actual content, separated from the preceding parts by a comma

Data URLs are best used in the following scenarios:

• For small images and icons (under 5KB)
• For SVG graphics that are used throughout a site
• For small, repeating background patterns
• When you want to reduce HTTP requests for small resources
• For embedding images in HTML emails
• For offline-capable web applications

For more detailed guidance, check our Best Practices page.

Data URLs should be avoided in these scenarios:

• For large images or files (over 5KB)
• For resources that change frequently
• For resources that would benefit from separate caching
• For content that needs to be indexed by search engines
• When SEO for images is important
• For resources shared across multiple pages (where caching would be beneficial)

Modern browsers don't have a specific size limit for Data URLs, but there are practical limitations:

• Internet Explorer 8 had a 32 KB size limit
• Some browsers may have limits in specific contexts (like URLs in CSS)
• Large Data URLs can impact performance by increasing HTML/CSS file size and parsing time
• Memory usage increases with large inline resources

As a best practice, it's recommended to limit Data URLs to resources under 5KB in size.

The choice between Base64 and URL encoding depends on the type of content:

• Base64 encoding is required for binary files like PNG, JPEG, GIF, etc.
• URL encoding is often more efficient for text-based files like SVG, HTML, CSS, etc.

For SVG files specifically, URL encoding typically results in a smaller string than Base64 encoding, making it the preferred choice for SVG Data URLs.

// URL-encoded SVG
data:image/svg+xml,%3Csvg xmlns=%22http://www.w3.org/2000/svg%22 width=%2224%22 height=%2224%22%3E...%3C/svg%3E

// Base64-encoded SVG
data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIyNCIgaGVpZ2h0PSIyNCI+...

Data URLs are supported in all modern browsers, including:

• Chrome
• Firefox
• Safari
• Edge
• Opera

However, there are some browser-specific limitations to be aware of:

• Internet Explorer 8 had a 32 KB size limit for Data URLs
• Some browsers restrict Data URLs in certain contexts for security reasons
• Data URLs for JavaScript may be blocked in some contexts

Data URLs have both positive and negative performance impacts:

Positive impacts:

• Reduce the number of HTTP requests, which can improve initial page load time
• Eliminate DNS lookups and connection overhead for small resources
• Ensure resources are available immediately with the HTML/CSS

Negative impacts:

• Base64 encoding increases file size by approximately 33%
• Increase HTML/CSS file size and parsing time
• Cannot be cached separately from the containing document
• Increase memory usage as part of the document

For optimal performance, use Data URLs for small, frequently used resources that don't change often, and use traditional URL references with proper caching for larger files.