Google Fonts: Language selection not working [duplicate]

Question

I am using the Google Font Roboto on quite a few client websites.

When you are customising the font on Google Fonts you have many language options to pick from:

• Greek (Supported by Roboto)
• Latin Extended (Supported by Roboto)
• Cyrillic (Supported by Roboto)
• Vietnamese (Supported by Roboto)
• Cyrillic Extended (Supported by Roboto)
• Greek Extended (Supported by Roboto)
• Latin (Supported by all Fonts)

For these client sites, I only need to use the Latin language, and don't need to load in any other languages.

However, when I load this font in and the Latin language defined, you can see all the other languages are also loaded in the generated stylesheet:

https://fonts.googleapis.com/css?family=Roboto:300&subset=latin

/* cyrillic-ext */
@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local('Roboto Light'), local('Roboto-Light'), url(https://fonts.gstatic.com/s/roboto/v15/0eC6fl06luXEYWpBSJvXCBJtnKITppOI_IvcXXDNrsc.woff2) format('woff2');
  unicode-range: U+0460-052F, U+20B4, U+2DE0-2DFF, U+A640-A69F;
}
/* cyrillic */
@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local('Roboto Light'), local('Roboto-Light'), url(https://fonts.gstatic.com/s/roboto/v15/Fl4y0QdOxyyTHEGMXX8kcRJtnKITppOI_IvcXXDNrsc.woff2) format('woff2');
  unicode-range: U+0400-045F, U+0490-0491, U+04B0-04B1, U+2116;
}
/* greek-ext */
@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local('Roboto Light'), local('Roboto-Light'), url(https://fonts.gstatic.com/s/roboto/v15/-L14Jk06m6pUHB-5mXQQnRJtnKITppOI_IvcXXDNrsc.woff2) format('woff2');
  unicode-range: U+1F00-1FFF;
}
/* greek */
@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local('Roboto Light'), local('Roboto-Light'), url(https://fonts.gstatic.com/s/roboto/v15/I3S1wsgSg9YCurV6PUkTORJtnKITppOI_IvcXXDNrsc.woff2) format('woff2');
  unicode-range: U+0370-03FF;
}
/* vietnamese */
@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local('Roboto Light'), local('Roboto-Light'), url(https://fonts.gstatic.com/s/roboto/v15/NYDWBdD4gIq26G5XYbHsFBJtnKITppOI_IvcXXDNrsc.woff2) format('woff2');
  unicode-range: U+0102-0103, U+1EA0-1EF9, U+20AB;
}
/* latin-ext */
@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local('Roboto Light'), local('Roboto-Light'), url(https://fonts.gstatic.com/s/roboto/v15/Pru33qjShpZSmG3z6VYwnRJtnKITppOI_IvcXXDNrsc.woff2) format('woff2');
  unicode-range: U+0100-024F, U+1E00-1EFF, U+20A0-20AB, U+20AD-20CF, U+2C60-2C7F, U+A720-A7FF;
}
/* latin */
@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local('Roboto Light'), local('Roboto-Light'), url(https://fonts.gstatic.com/s/roboto/v15/Hgo13k-tfSpn0qi1SFdUfVtXRa8TVwTICgirnJhmVJw.woff2) format('woff2');
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2212, U+2215, U+E0FF, U+EFFD, U+F000;
}

It was my understanding that defining &subset=latin loads in only that language, so why are all the languages being loaded in here?

Answer 1

I am copying this answer by Daryl Teo as he seems to provide the correct answer to your question.

The trick lies with this optimization:

unicode-range: U+0400-045F, U+0490-0491, U+04B0-04B1, U+2116;

With this, the browser knows whether it needs to download the font, depending on the characters it just loaded in the html.

See here which browsers have full support for this.

Earlier browsers, e.g., Firefox < 44 and Safari < 10, ignored (parts) of the spec, or it needed to be enabled, therefore Google Fonts has to serve it a minimal font-face specification.