Ligatures are important as they guarantee a better flow of legibility of the text and better readability of a word when some characters clash.
In fact, they are usually added, or better designed, when two letters are joined as a single one. This usually happens when two letters have ascenders, like ff, ft, that clash together producing a rather unpleasant result.
Even the ampersand symbol is born as a ligature, when the et in the handwritten Latin text where combined.
So, how we can apply them to the Web?
CSS3 standard introduces the property font-variant-ligatures to precisely address the matter through numerous values, such as:
Common-ligatures - no-common-ligatures
Discretionary-ligatures - no-discretionary-ligatures
Historical-ligatures - no-historical-ligatures
Contextual - no-contextual
Normal and none.
Each property can be declared with multiple values, containing all of the preceding declared values..
For example, if you like discretionary but not historical ligatures, you can write...