# This is why you must setup the @font-face declarations correctly

Browsers will inject a User-Agent style sheet that specifies H1-H6 to have "font-weight:bold;" in the CSS:

      h1 {
          display: block;
          font-size: 2em;
          -webkit-margin-before: 0.67em;
          -webkit-margin-after: 0.67em;
          -webkit-margin-start: 0px;
          -webkit-margin-end: 0px;
          font-weight: bold;
      }

H1 and H3 "Museo 100" with UA-Style sheet applied:

# This is a Museo 100 headline

Unless you fix the font-weight declaration it will always be bold. It is not enough to assume the font weight will only be the weight specified

      h1.fixed {
        font-weight: normal;
      }

# This is a Museo 100 headline

# Boostrap does not suffer from this problem

Because bootstrap adds a reset "font-weight:500" declaration to H1-H6 the headlines are not effected in the same way. Font-Weight:500 simply fails (unless specified by your font declaraion to use a bolder font) and we are left with intended font weight.

# This is a Museo 100 headline