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:

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;
  }

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.