As Osvaldo comments, it would be helpful to know just why you think you need to know this.
When a user visits a web page, the browser will parse the page using whatever encoding the server tells it to use (via the HTTP
Content-Type header or the corresponding HTML
The only time the browser default encoding matters is when the server doesn't specify the encoding (so that the browser has to guess), or possibly when the server specifies an oddball encoding that the browser doesn't understand (but that's pretty unusual, since browsers tend to know quite a few encodings).
For historical reasons, the default encoding is often ISO-8859-1, since that was the most commonly used encoding back in the very early days of the web, when authors were not so careful about specifying encodings.
Many browsers will also not default to any specific encoding, but will instead try to guess the correct encoding by heuristic analysis if none is explicitly specified. For example, my Firefox default encoding is set simply to "Auto-Detect / Universal"; there are also other auto-detect setting that prefer one encoding (or encodings common in one region / language) over others in ambiguous cases.
But for authors, there's really no reason not to just use UTF-8 these days. Every browser supports it, and, being a Unicode encoding, it contains essentially all characters found in every other encoding.