doc: clarify conditional exports guidance

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, include
+a `"defaut"` condition where possible. Always providing a `"default"` condition
+ensures that any unknown JS environments are able to use a universal
+implementation where possible, which helps avoid JS environments from having to
+pretend to be existing environments to support packages with conditional
+exports. Where possible, `"node"` and `"default"` conditions can be preferable
+to `"browser"` conditions.
 
 #### 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",
   }
 }
 ```