https://www.w3schools.com/css/css_border.asp
CSS Borders
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(공식문서)
https://developer.mozilla.org/ko/docs/Web/CSS/border
border - CSS: Cascading Style Sheets | MDN
CSS border 단축 속성은 요소의 테두리를 설정합니다. border-width, border-style, border-color의 값을 설정합니다.
developer.mozilla.org