Free webp to jpg
EN
Image MIME Reference
ICO

Windows Icon (Legacy MIME)

image/x-icon
VS

Progressive JPEG (Legacy IE Alias)

image/pjpeg
PJPEG

A complete technical comparison of Windows Icon (Legacy MIME) and Progressive JPEG (Legacy IE Alias) — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.

Equal browser support

Feature Support

Feature ICO PJPEG
Transparency (Alpha) Yes ✕ No
Animation Support ✕ No ✕ No
Progressive Loading ✕ No Yes
HDR Support ✕ No ✕ No
EXIF Metadata ✕ No Yes
ICC Color Profile ✕ No Yes
Windows Icon (Legacy MIME)
image/x-icon
Extension
.ico
Container ICO Directory Structure
Compression Uncompressed (DIB/BMP) / Deflate (Embedded PNG)
Algorithm
NoneDeflate (via PNG)
Color Depth 1-bit, 4-bit, 8-bit, 24-bit (True Color), 32-bit (RGBA)
Developed by Microsoft Corporation
Released 1985
Magic Bytes
00 00 01 00
Full ICO reference →
Progressive JPEG (Legacy IE Alias)
image/pjpeg
Extension
.jpeg.jpg.pjpeg.pjp.jfif
Container JPEG (ISO/IEC 10918-1)
Compression Lossy (DCT)
Algorithm
Discrete Cosine Transform (DCT)
Color Depth 8-bit, 12-bit
Developed by Joint Photographic Experts Group / Microsoft (Alias)
Released 1992
Magic Bytes
FF D8 FF E0 (JFIF) / FF D8 FF E1 (Exif)
Full PJPEG reference →

Browser Support Comparison

Browser ICO PJPEG
Chrome Chrome
✓ 1
✓ All
Firefox Firefox
✓ 1
✓ All
Safari Safari
✓ 1
✓ All
Edge Edge
✓ 12
✓ All
IE IE (Legacy)
✓
✓
ICO

ICO Strengths

  • Universally supported by every web browser in existence for website favicons
  • Packs multiple resolutions into a single file, eliminating the need to serve different files for the taskbar vs. the desktop
  • Modern support for embedded PNGs (since Windows Vista) allows for excellent 32-bit transparency (alpha channels) and drastically reduced file sizes
Limitations
  • Internal file structure is convoluted due to decades of legacy cruft, such as the requirement for BMP data to lack a standard file header but double the height property for an archaic XOR/AND mask
  • Larger file size compared to a single optimized PNG or SVG, as it literally stores the same image multiple times at different scales
  • The MIME type landscape is highly fragmented (x-icon vs. vnd.microsoft.icon), causing occasional caching or strict-MIME validation errors on restrictive servers
PJPEG

PJPEG Strengths

  • Progressive JPEGs (which this MIME type attempted to describe) provide excellent perceived loading speeds on slow connections by rendering a low-resolution preview first
  • Files sent with this MIME type are fully backwards compatible with any standard JPEG decoder
Limitations
  • It is a non-standard MIME type that violates IANA specifications
  • Causes strict file upload validation scripts to fail if not explicitly whitelisted or mapped
  • Creates unnecessary confusion, as the official image/jpeg MIME type already fully encompasses progressive encoding

When to choose which format

ICO
Use ICO when…
  • Website Favicons (favicon.ico)
  • Windows Application Executable Icons (.exe)
  • Desktop Shortcut Icons
PJPEG
Use PJPEG when…
  • Legacy Internet Explorer File Uploads
  • Progressive Image Rendering (Historical context)

More comparisons

ICO vs ACES → PJPEG vs ACES → ICO vs APNG → PJPEG vs APNG → ICO vs AVCI → PJPEG vs AVCI → ICO vs AVCS → PJPEG vs AVCS → ICO vs AVIF → PJPEG vs AVIF → All comparisons →
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