Image MIME Reference
ICO
Windows Icon (Legacy MIME)
image/x-icon
VS
JPEG XS Codestream
image/jxsc
JPEG-XS-Codestream
A complete technical comparison of Windows Icon (Legacy MIME) and JPEG XS Codestream — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
ICO wins on browser support
Feature Support
| Feature | ICO | JPEG-XS-Codestream |
|---|---|---|
| Transparency (Alpha) | Yes | Yes |
| Animation Support | ✕ No | ✕ No |
| Progressive Loading | ✕ No | ✕ No |
| HDR Support | ✕ No | Yes |
| EXIF Metadata | ✕ No | ✕ No |
| ICC Color Profile | ✕ No | ✕ No |
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
JPEG XS Codestream
image/jxsc
Extension
.jxsc.jxs
Container
None (Raw Codestream)
Compression
Lossy (Visually Lossless) / Mathematically Lossless
Algorithm
JPEG XS (ISO/IEC 21122-1)
Color Depth
8-bit, 10-bit, 12-bit, 14-bit, 16-bit
Developed by
Joint Photographic Experts Group (JPEG) / intoPIX
Released
2019
Magic Bytes
FF 10
Browser Support Comparison
| Browser | ICO | JPEG-XS-Codestream |
|---|---|---|
Chrome
|
✓ 1
|
✕ |
Firefox
|
✓ 1
|
✕ |
Safari
|
✓ 1
|
✕ |
Edge
|
✓ 12
|
✕ |
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
JPEG-XS-Codestream
JPEG-XS-Codestream Strengths
- Eliminates container overhead, maximizing parsing speed and minimizing latency to fractions of a millisecond
- Highly optimized for direct hardware implementation in FPGAs and ASICs
- Perfect for continuous data streaming over IP networks without needing to establish file headers
Limitations
- Lacks standard container metadata (EXIF, XMP, ICC profiles), meaning color space and orientation data must be handled externally or out-of-band
- Zero support in web browsers or standard consumer operating systems
- Often requires specialized broadcast hardware or custom software libraries to decode
When to choose which format
ICO
Use ICO when…
- Website Favicons (favicon.ico)
- Windows Application Executable Icons (.exe)
- Desktop Shortcut Icons
JPEG-XS-Codestream
Use JPEG-XS-Codestream when…
- Hardware Decoding Pipelines (FPGA/ASIC)
- Live Broadcast Infrastructure
- Video over IP Transport
- Embedded Camera Systems