pagespeed UrlValuedAttribute a href image;
pagespeed UrlValuedAttribute a data-src image;
pagespeed UrlValuedAttribute custom src image;
pagespeed UrlValuedAttribute custom href image;
pagespeed UrlValuedAttribute custom data-src image;
pagespeed UrlValuedAttribute img data-src image;
All images should be converted to webp in supporting browsers, but in my testing only the ones on the img tag are, not the ones on the a tag.
Update: Huibao figured it out: my custom tag wasn't closed, and the a tag has a child.