Is it safe to use the yuva420 pix_format option with ffmpeg to encode webm videos?
YUV color encoding with 4:2:0 chroma subsampling is the only widely safe pixel format for web playback, although for certain codecs, other pixel formats may be supported.

FFmpeg auto-selects the pixel format for the output as not all encoders support all pixel formats. It will select the pixel format which best preserves the source bit depth, chroma sampling scheme and the color encoding scheme, from among the formats supported by the encoder. PNGs have RGB pixel format and until two years ago, ffmpeg did not support encoding RGB formats for the VPx encoders, so it would select a YUV format with full chroma (4:4:4). Since RGB became supported in FFmpeg for VPx, it will select a RGB variant like gbrp.

Auto-selection is overriden by manually specifying -pix_fmt.

YUV streams can be more efficiently compressed than RGB.