doc: clarify conditional exports guidance

PR-URL: #34306
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>

Author: guybedford

doc/api/esm.md

@@ -456,16 +456,24 @@ Conditional exports can also be extended to exports subpaths, for example:
 "exports": {
 ".":"./main.js",
 "./feature": {
-"browser":"./feature-browser.js",
+"node":"./feature-node.js",
 "default":"./feature.js"
  }
  }
 }
 ```
 
 Defines a package where `require('pkg/feature')` and `import 'pkg/feature'`
-could provide different implementations between the browser and Node.js,
-given third-party tool support for a `"browser"` condition.
+could provide different implementations between Node.js and other JS
+environments.
+
+When using environment branches, always include a `"default"` condition where
+possible. Providing a `"default"` condition ensures that any unknown JS
+environments are able to use this universal implementation, which helps avoid
+these JS environments from having to pretend to be existing environments in
+order to support packages with conditional exports. For this reason, using
+`"node"` and `"default"` condition branches is usually preferable to using
+`"node"` and `"browser"` condition branches.
 
 #### Nested conditions
 
@@ -479,11 +487,11 @@ use in Node.js but not the browser:
 {
 "main":"./main.js",
 "exports": {
-"browser":"./feature-browser.mjs",
 "node": {
 "import":"./feature-node.mjs",
 "require":"./feature-node.cjs"
- }
+ },
+"default":"./feature.mjs",
  }
 }
 ```