The short answer, is "all". Or, more precisely, "all modern browsers".
A more complete answer will list browsers in order of priority (often arranged in what we call a browser matrix). And these priorities are determined by the browser types that visit your site. If you don't have such data (as might be the case with a new site or a site that has not been collecting analytics), your browser matrix will prioritize browsers by their market share. Over time, as you start to collect actionable web analytics data, your browser matrix may change a little if you discover that your site is visited regularly by a combination of browsers that differs from the generic market share statistics.
The above simply means that your browser targeting strategy is likely to consist of a three step process:
- Start with market share based priorities, tweaked as required to match reasonable expectations about user types, device types, user locations, etc
- Refine starting priorities with data collected from your own site
- Revisit the previous step regularly and refine as required
Today, since most modern browsers do well with standards compliance, most design features (as long as they are themselves standards compliant), will work acceptably in most commonly used browsers without the need for excessive browser-specific tweaks.