<img src="a"
srcset="
 should-not-format  400w 100h,
       should-not-format  500w 200h
"
 alt=""/>

<img src="a"
srcset="
 should-not-format ,, should-not-format 0q,,,
"
 alt=""/>

<img src="a"
srcset=",,,"/>

<img src="a"
srcset="   "/>
