html-lint.css

/* Find images with missing alt attributes */
img:not([alt]) {
  outline: 2px dotted red;              
}

/* Potential tabindex problems */
[tabindex]:not([tabindex="0"], [tabindex="-1"]) {
  outline: 2px dotted red;
}

/* Links that go nowhere */
a:is(:not([href]), [href=""], [href="#"]) {
  outline: 2px dotted red; 
}

/* Invalid list elements */
:is(ul,ol) > *:not(li) {
  outline: 2px dotted red;
}

/* Images without width and height */
img:not([width]):not([height]) {
  filter: blur(20px);
}

/* Headers out of order (i.e. h2 before h1, etc.)
   Result: dotted blue outline
*/
h2 ~ h1,
h3 ~ h1,
h4 ~ h1,
h5 ~ h1,
h6 ~ h1,
h3 ~ h2,
h4 ~ h2,
h5 ~ h2,
h6 ~ h2,
h4 ~ h3,
h5 ~ h3,
h6 ~ h3,
h5 ~ h4,
h6 ~ h4,
h6 ~ h5 {
  outline: 2px dotted red;
}