William Horvath focused on maintaining cross-version Windows compatibility in the libsdl-org/SDL repository, addressing compilation issues with legacy Windows SDKs. He resolved a bug that prevented building on pre-Vista toolchains by manually declaring missing COM interfaces such as IFileDialog, IFileDialog2, and IFileOpenDialog, along with their associated flags and types. To ensure runtime compatibility, William implemented dynamic loading of SHCreateItemFromParsingName from shell32.dll, allowing newer APIs to function on older systems. His work, using C programming and Windows API expertise, demonstrated a deep understanding of cross-platform development and backward compatibility, though it was limited to a single targeted bug fix.