Blob URLs, Data URLs, and WebP Conversion in the Browser

Not every image on a web page is loaded from a normal https:// file URL. Some pages use blob URLs, data URLs, generated previews, canvas output, or temporary object URLs. A browser-based WebP conversion tool can handle some of these cases, but the behavior depends on how the page created the image and what the browser allows.

Understanding these URL types helps set realistic expectations for right-click converters and extension workflows.

For a browser extension, this topic also belongs in the product boundary documentation. GetWebP's context-menu guide says right-click conversion is for direct image elements and may not work on protected, iframe, or cross-origin cases. The security overview explains the related privacy boundary: conversion can be local even when the browser still has to fetch the selected image from its original origin.

Treating blob: and data: sources like ordinary files with unusual prefixes hides the important risks. Browser conversion docs should preserve source type, page state, expiration behavior, logging policy, output dimensions, and fallback path so users understand whether a failure came from browser context, privacy rules, or actual converter support.

What a Blob URL Represents#

A blob URL is a temporary URL that points to data held by the browser. MDN's URL.createObjectURL() documentation explains how web pages can create object URLs for Blob or File objects.

Blob URLs often look like this:

blob:https://example.com/2b3b1b2f-...

They may be used for image previews, downloads generated in the browser, drag-and-drop workflows, or files selected from the user's machine. The URL may not keep working outside the page session, and it may not be fetchable in the same way as a public image file.

Do not treat the blob URL string as a durable asset identifier. It is a handle created by the page or browser for the current context. A support ticket that only says blob:https://example.com/... usually does not tell the developer what original file, preview, or generated output was behind it.

What a Data URL Represents#

A data URL embeds the file data directly inside the URL. MDN's data URL reference describes the syntax and behavior.

A data URL may look like:

data:image/png;base64,iVBORw0KGgo...

Data URLs can be convenient for tiny images, inline previews, and generated assets, but large data URLs are awkward to inspect and can make pages heavy. For conversion tools, the data may already be present, but the tool still needs to decode it, validate it, and write a new file.

Data URLs also deserve careful logging rules. A full data URL can contain the entire image. If the image is private, copying the full string into a log, analytics event, or support email can leak the same content as uploading the file.

Why These URLs Matter for WebP Tools#

For normal image URLs, a tool can often fetch the image, decode it, convert it, and save the result. Blob and data URLs are more context-dependent. They may only exist inside the current tab, may be tied to a page's JavaScript, or may represent user-selected local files.

A browser extension or web app should handle these cases carefully:

• confirm the data is actually an image
• respect browser security boundaries
• avoid leaking local file data
• choose a clear output filename
• show a useful error if the source expires
• avoid assuming the original URL can be reused later

The tool should treat temporary URLs as temporary.

The implementation and support docs should distinguish these cases:

Blob URLs Can Expire#

Pages can revoke blob URLs when they no longer need them. The browser can also discard page state when a tab is closed or reloaded. If a user right-clicks a blob image after the page changes, the converter may no longer have access to the underlying bytes.

This is normal browser behavior. A good tool should fail clearly rather than pretending the image was saved.

If a workflow depends on blob images, convert them while the page and preview are still active.

A useful failure message is specific: "temporary image source expired" tells the user to regenerate the preview or reselect the file. "Conversion failed" does not.

Data URLs Can Be Very Large#

Data URLs include the content inline, so they can become long and hard to manage. A small icon may be fine. A large product image encoded into a data URL can be inefficient and difficult to debug.

When converting from a data URL, inspect output size and visual quality just as you would with a file URL. The source may already be compressed, resized, or generated from a canvas.

Do not assume a data URL is the original image. It may be a preview.

For privacy review, write down whether the tool logs the data URL, truncates it, hashes it, or avoids recording it entirely. The safest default for support and telemetry is to record only the source type, MIME type, approximate size, and error category.

Canvas and Cross-Origin Limits#

Some browser conversion workflows draw an image to a canvas before exporting WebP. Cross-origin rules can restrict reading pixels from a canvas if the source image is not allowed for that use. This protects users and sites from unintended data access.

If a page displays an image but the converter cannot read its pixels, the issue may be security policy rather than file format. The tool should distinguish this from unsupported input or network failure.

The extension error guide is the right kind of product surface for this distinction. Users need to know whether they should retry, use the extension popup with a local file, export from the source app, or leave the image unchanged.

Keep Privacy Language Concrete#

Blob and data URLs may contain user-selected files or generated content. A local-first tool should state whether those bytes stay on the device during conversion. If the tool uploads image data, users should know.

For privacy-sensitive workflows, avoid tools that are vague about temporary image data. The fact that a URL starts with blob: or data: does not make it safe to transmit.

A concrete privacy note can be short:

Temporary source tested: blob preview generated by CMS
Image bytes uploaded to converter service: no
Full blob/data URL logged: no
Source type recorded: blob
Failure case: expired preview produced visible error
Recommended fallback: export original from CMS, convert local copy

That is the level of detail a team can evaluate.

Test Real Cases#

A practical browser-conversion test set should include:

• normal https:// image
• responsive image candidate
• lazy-loaded image
• blob URL preview
• data URL icon
• large data URL image
• canvas-generated image
• expired blob URL case

This helps separate supported behavior from edge cases.

Keep the expected result beside each case:

normal https image -> saved WebP or HTTP error
responsive image -> documented selected candidate saved
blob preview -> saved only while preview is active
expired blob URL -> clear temporary-source error
data URL icon -> saved without logging full source
large data URL image -> saved or size/decoder error with no full URL log
canvas-generated image -> saved only when readback is allowed
protected viewer -> unsupported-source message

Blob URLs and data URLs are common in modern web apps. WebP conversion tools can support them well when they respect browser context, temporary lifetimes, security boundaries, and clear user messaging.