CSS

css keep image ratio

img {
  object-fit: cover;
  width: 100px;
  height:100px;
}

keep aspect ratio of image css

img {
    display: block;
    max-width: 100%;
    max-height: 100%;
    width: auto;
    height: auto;
}

html image keep aspect ratio

Don't set height AND width. Use one or the other and the correct aspect ratio will be maintained.

css keep aspect ratio image

.image-full {
    background: url(...some image...) no-repeat;
    background-size: cover;
    background-position: center center;
}