Help with self-hosting fonts on *.elm-lang.org

That sounds accurate - if I look into the file returned for Internet Explorer it only has a small number of format('woff') fonts like

@font-face {
  font-family: 'Source Code Pro';
  font-style: normal;
  font-weight: 400;
  src: local('Source Code Pro Regular'), local('SourceCodePro-Regular'), url(https://fonts.gstatic.com/s/sourcecodepro/v11/HI_SiYsKILxRpg3hIP6sJ7fM7PqlPevQ.woff) format('woff');
}

instead of a much larger number of format('woff2') ones that are also split by Unicode range, like

/* latin */
@font-face {
  font-family: 'Source Code Pro';
  font-style: normal;
  font-weight: 400;
  src: local('Source Code Pro Regular'), local('SourceCodePro-Regular'), url(https://fonts.gstatic.com/s/sourcecodepro/v11/HI_SiYsKILxRpg3hIP6sJ7fM7PqlPevW.woff2) format('woff2');
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
}