TLDR :

img {
  max-width: 100%;
}
img[width] {
  width: auto; /* Defer to max-width */
}
img[width][height] {
  height: auto; /* Preserve aspect ratio */
}

/* Let SVG scale without boundaries */
img[src$=".svg"] {
  width: 100%;
  height: auto;
  max-width: none;
}