CSS3 Essentials  
Border
  1. Outer edge of the padding and inner edge of margin.
  2. Default border size is zero and invisible.
  3. Border properties/Border longhand
    1. <border-width> | <border-style> | <border-color> 
  4. Border shorthand: 
    1. Syntax: border: border-width border-style border-color   ;
    2. Example: .disk { border: 3x solid blue; }
  5. Border longhand 
    1. Border shorthand + {keyword}
      1. <border-top> | <border-right> | <border-bottom> | <border-left>
      2. <border-width> | <border-style> | <border-color> 
    2. Border shorthand + {keyword} + {keyword}
      1. <border-top-width> | <border-top-style> | <border-top-color> | <border-left-width> | ...
  6.  Border radius
    1. border-radius: top right bottom left;
    2. Example: 
      1. border-radius: 20px;
      2. border-radius: 20px 30px;
      3. border-radius: 20px 30px 40px;
      4. border-radius: 20px 30px 40px 50px;
    3. Border radius longhand options
      1. <border-top-left-radius> | <border-top-right-radius> | <border-bottom-left-radius> | <border-bottom-right-radius>
  7.  Create elliptical corners
    1. div { border-radius: 10px / 20px; }
    2. div { border-radius: 10px 20px / 30px 40px; }
  8. Border Image
    1. Require when complex design for border not like solid color.
    2. Background image is the bottom image.
    3. Properties
      1. <border-image-source>
      2. <border-image-slice>
      3. <border-image-repeat>
  9. Ref:
    1. https://developer.mozilla.org