Вы также можете играть с градиентом и background-clip (см. Комментарии в CSS)
button {
vertical-align: top;
border: 5px solid transparent;/* give extra space for gradients colors */
font-size: 2.5rem;
margin: 0.25em;
padding: 0.5em 2em;
background: linear-gradient(#333, #333),/* black turned into gradient to hold with background-clip and hide the 2 color gradient under it */
linear-gradient(/* 2 colors to draw under the borders also via background-clip*/
to bottom left,
rgb(230, 83, 0) 50%,
gray 51%,
rgb(0, 166, 224) 40%
)
no-repeat center center;
background-clip:
padding-box, /* drawn inside including padding area */
border-box;/* drawn also under borders */
background-size:
100% 100%,
110% 150%;/* must be bigger than 100% so it include also borders, else it repeats */
color: white;
box-shadow: 0 0 2px 2px black, inset 0 0 2px black;/* did you want this too ? */
}
<button>BUTTON</button> <button> TO</button> <button> PLAY</button>
Если вы думаете, что это слишком много, у вас также есть border-image .