If called in HTML/CSS the font-family name is converted to an mPDF font-family name (seeFont names) e.g.

FIXME

The font requested (including style) is checked to see if it is available to mPDF: set by the array$available_unifonts:

1. If the font[style] exists - selected e.g.trebuchetms['B']
2. If the font[nostyle] exists - selected e.g.trebuchetms['R']
3. Looks up the font-family in the three arrayssans_fonts, serif_fonts, andmono_fonts defined inmpdf_config.php , in this case looking for ‘trebuchetms’. If found,substitutes a font of similar type (sans-serif, serif, or mono) - the first font in thesans_fonts, serif_fonts,ormono_fonts arrays is used.
4. If no font has yet been selected, the first font in the array$available_unifonts * is selected

* The array $available_unifonts is initially derived from$this->fontdata in theconfig_fonts.php file. (trebuchetms['B'] will be converted to ‘trebuchetmsB')

If core fonts only are specified by using$mpdf = new \Mpdf\Mpdf(['mode' => 'c']), then all font requests will besubstituted by Arial/Helvetica, Times or Courier i.e. the core PDF fonts. mPDF determines whether the requested fontis a sans-serif, serif or monospace font (as above), and substitutes accordingly. 

If some circumstances, individual characters are replaced by glyphs from another font - seecharacter (font) substitution.