W3C-Validator | Pure Python command line for HTML validation using W3C | Validation library

I think you need to add a space between the media query and the size: change (max-width:768px)100vw to (max-width:768px) 100vw.

From MDN:

sizes defines a set of media conditions (e.g. screen widths) and indicates what image size would be best to choose, when certain media conditions are true — these are the hints we talked about earlier. In this case, before each comma we write:

1. A media condition ((max-width:600px)) — you'll learn more about these in the CSS topic, but for now let's just say that a media condition describes a possible state that the screen can be in. In this case, we are saying "when the viewport width is 600 pixels or less".
2. A space
3. The width of the slot the image will fill when the media condition is true (480px)

https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images#resolution_switching_different_sizes