Some source code fonts support ligatures, allowing to represent some common Ascii characters combinations with a special glyph instead — e.g. the comparison operator >= being substituted by ⩾, the inequality comparator != by ≠, and the Ascii rightward arrow -> by the → glyph.
Needless to say, ligatures add beauty to source code as Ascii combinations are shown with the proper symbols they intend to represent (mathemical, logical, or otherwise).
Using Ligatures With Highlight
Here's an animated screenshot showing an Elixir code example being highlighted in Highlight GUI, using both a font that supports ligatures (Fira Code) and one that doesn't (Inconsolata):
All that was required to enable ligatures in the GUI was to select a font that supports them (in the above example, Fira Code). Since the font settings in Highlight GUI are carried over to the converted HTML document, ligatures will also be available in the output document.
The Ligature Fonts Plugin
Since v3.45, Highlight ships with a plugin to enable font ligatures in HTML/XHTML output:
Bare in mind that not all fonts support the same ligatures sets, and that some fonts might work better with some languages instead of others (for example, Hasklig was designed with Haskell in mind).
One of the typical ligatures combinations you should watch out for are <= and =>, which some fonts will treat as comparison operators (⩽ and ⩾) while others will treat them as arrows (⇐ and ⇒). So much so, that some fonts are avaible in two different versions for these ligatures (e.g. Fixedsys Excelsior).
So, you should always test your source code against various ligatures fonts before publishing your highlighted code, to ensure that you pick the font which works best with that language.