2 Specified code compatibility
source | link

HTML/CSS (Chrome only), 80 + 4 = 84 bytes

tt:after{content:"--:--";animation:a 1s infinite}@keyframes a{to{content:"12:00"
<tt>

Edit: The "content" attribute is not animatable by the CSS specification, but is on Chrome desktop browser.

HTML/CSS, 80 + 4 = 84 bytes

tt:after{content:"--:--";animation:a 1s infinite}@keyframes a{to{content:"12:00"
<tt>

HTML/CSS (Chrome only), 80 + 4 = 84 bytes

tt:after{content:"--:--";animation:a 1s infinite}@keyframes a{to{content:"12:00"
<tt>

Edit: The "content" attribute is not animatable by the CSS specification, but is on Chrome desktop browser.

1
source | link

HTML/CSS, 80 + 4 = 84 bytes

tt:after{content:"--:--";animation:a 1s infinite}@keyframes a{to{content:"12:00"
<tt>