Personally I think it is like a good manner to always add width and height attributes to img elements on point where the img-element is generated. That is the place where I might know for sure which exact image-file I’m putting in. CSS is far away from the actual image-file.
On top the browsers use this extra info about the image size to calculate aspect-ratio before loading the image file - and before CSS parsing.
For this case the emojis are used in many different places - and sometimes defining sizes via CSS will be skipped accidentally.
E.g. see crawler view of this very same topic on PageSpeed
