oh those are wonderful! i love the border-image attribute so much.
ive done a couple of them for my Mega Man Battle Network and Mega Man Shrines.
would love to contribute some of them here!

classic megaman, took the sprites directly from Mega Man 10.
.menuborder {
background: #000000;
border-image-slice: 6 6 6 6;
border-image-width: 6px 6px 6px 6px;
border-image-outset: 0px 0px 0px 0px;
border-image-repeat: repeat repeat;
border-image-source: url("mm_menuborder.png");
}

battle network dialogue text, took it from Battle Network 4.
.dialogue {
background: #ffffe7;
border-image-slice: 14 6 6 81;
border-image-width: 14px 6px 6px 82px;
border-image-outset: 12px 0px 0px 0px;
border-image-repeat: stretch stretch;
border-image-source: url("img/bn_dialogue.png");
border-style: solid;
}
there's a couple others from my Battle Network shrine, but i feel those could be improved before being used by others.
hope these are neat additions to your borders, may use some for later stuff, thanks for sharing!