If called in HTML/CSS the font-family name is converted to an mPDF font-family name (seeFont names in 5.x) 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 afont of similar type (sans-serif, serif, or mono) - the first font in thesans_fonts, serif_fonts, ormono_fontsarrays 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.phpfile. (trebuchetms['B'] will be converted to ‘trebuchetmsB' The array can be altered when certain languages are defined e.g.when using lang markup in the HTML code:

If ‘ar’ (arabic) is set as the language, the default settings inconfig_cp.php define arestricted set of fonts which can be selected. In the example case, mPDF will look fortrebuchetmsB and if notavailable,trebuchetms

If core fonts only are specified by using$mpdf = new \Mpdf\Mpdf('c'), then all font requests will be substituted byArial/Helvetica, Times or Courier i.e. the core PDF fonts. mPDF determines whether the requested font is 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.