Or cut and paste it into your css.

      
/*
  tachyons-box-sizing.css
*/
nav,
main,
section,
article,
aside,
header,
footer,
div,
details,
summary,
form,
fieldset,
textarea,
input[type="text"],
input[type="tel"],
input[type="url"],
input[type="email"],
input[type="search"],
input[type="number"],
input[type="password"],
.border-box {
  box-sizing: border-box;
}