<style>
.box1 {
position: relative;
width: 600px;
height: 360px;
left: calc(50% - 300px);
border: 1px solid blue;
border-width: 1px 0 0 1px;
background:
repeating-linear-gradient(transparent 14px, red 14px 15px, transparent 15px 29px,blue 29px 30px,transparent 0 44px),
repeating-linear-gradient(90deg,transparent 14px, red 14px 15px, transparent 15px 29px,blue 29px 30px,transparent 0 44px);
}
</style>
<div class="box1">
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