WebAssembly AVIF Decoders in Older Browsers

Using WebAssembly (Wasm) to decode AVIF images in legacy browsers enables modern image compression support where native decoding is absent. However, this polyfill strategy introduces major technical trade-offs. Older browsers often lack the necessary browser APIs and WebAssembly extensions required for efficient software decoding, leading to severe performance bottlenecks, high memory consumption, and degraded user experience.

Lack of Baseline WebAssembly Support

Browsers released before 2017—such as Internet Explorer 11 and older mobile browsers—do not support WebAssembly natively. In these environments, a Wasm-based AVIF decoder cannot run at all unless compiled down to asm.js, a fallback that is significantly slower, larger in file size, and often impractical for production environments.

Absence of SIMD and Multithreading

Modern software-based AVIF decoding relies heavily on two WebAssembly extensions to achieve acceptable speeds:

Older browsers with early WebAssembly implementations lack support for Wasm SIMD and cross-origin isolated SharedArrayBuffer instances. Without these capabilities, decoders are forced to run in single-threaded scalar mode, resulting in decoding times that can be up to ten times slower than native decoding.

Main Thread Contention and Layout Delay

When images are decoded natively by the browser, decompression occurs asynchronously off the main thread with optimized platform pipelines. In contrast, running a Wasm decoder requires JavaScript intervention to fetch the binary, pass the image buffer to the Wasm instance, and draw the resulting pixel data onto an HTML5 <canvas> or convert it into a Blob URL. If workers are unsupported or misconfigured, this computation runs on the main thread, causing interface freezes, high Total Blocking Time (TBT), and poor Largest Contentful Paint (LCP) scores.

Memory Overhead and Resource Leaks

Decompressing high-resolution AVIF files requires allocating raw RGBA pixel arrays in the WebAssembly linear memory space before rendering. Older devices running outdated browsers typically operate under tight memory constraints. The memory footprint of the Wasm module itself, combined with large intermediate buffers, can trigger out-of-memory crashes on low-end mobile devices and legacy operating systems.

Bundle Size and Network Overhead

Using a client-side decoder requires downloading both the JavaScript glue code and the compiled WebAssembly binary (such as a compiled version of libavif or dav1d) before any images can be displayed. This payload often ranges from 150 KB to over 500 KB compressed. For pages with few images, the network overhead of fetching the decoder often exceeds the bandwidth saved by using AVIF over legacy formats like WebP or JPEG.

Increased Battery Consumption

Hardware acceleration is unavailable to WebAssembly decoders. Relying entirely on software CPU decoding for image rendering causes extended spikes in CPU usage, which drains device battery rapidly—especially on older, less power-efficient hardware.