A ligature is a single character formed from two consecutive characters, either for clarity or for stylistic reasons. Ligatures can be used in web documents through HTML character references. This article contains a chart of the ligatures available to web authors, and discusses support for ligatures in common fonts and search engines.
References for ligatures
The chart below shows the character entity references or numeric character references for various ligatures available to web authors. Depending on the fonts installed on your system, some of these may not be shown (or may be displayed as boxes or question marks). Of the fonts commonly used on the web, only Palatino contains all of these ligatures. However, seven of the ligatures are present in all of the common web fonts — the AE, ae, OE, oe, ss/sz, fi, and fl ligatures — and these may safely be used by web authors, without fear of incorrect display.
Ligatures and search engines
From a theoretical standpoint, search engines should regard ligatures and the characters from which they are formed as equivalent. Unfortunately, at the time of writing none of the major search engines (Google, Yahoo, MSN, and Ask) do so. This limits the usefulness of ligatures on the web, especially for authors who consider search engine optimisation to be important.