A MIDI file is a script, not a performance. The same file can sound like a cheap toy or a concert grand, depending on the SoundFont it meets. A SoundFont bundles real recorded samples of an instrument with the rules for playing them.
SF2 and SF3
SF2 stores raw 16-bit samples; SF3 stores the same structure with compressed audio inside, at a fraction of the size and nearly identical sound. General MIDI fonts cover every instrument a file might ask for; specialist fonts — a single superb piano, a vintage electric piano — sound better inside their own territory.
Choosing well
For piano transcriptions, pick a dedicated piano font and judge it on the middle octave, where most music lives. For mixes with several parts, a full GM font keeps every channel sensible. Size is a rough quality signal: under 3 MB usually means synthetic loops; 30–150 MB usually means real recordings.
Rendering in the browser
You don't need a DAW to audition a font. Load your MIDI, drop in an SF2 or SF3, and render to MP3 entirely in the tab — the samples are mixed offline and encoded locally. Swap fonts, re-render, compare: the whole A/B takes less time than opening a plugin.