docs: Add AX design principles (#643)

Author: natorion

README.md

@@ -7,7 +7,7 @@ control and inspect a live Chrome browser. It acts as a Model-Context-Protocol
 (MCP) server, giving your AI coding assistant access to the full power of
 Chrome DevTools for reliable automation, in-depth debugging, and performance analysis.
 
-## [Tool reference](./docs/tool-reference.md) | [Changelog](./CHANGELOG.md) | [Contributing](./CONTRIBUTING.md) | [Troubleshooting](./docs/troubleshooting.md)
+## [Tool reference](./docs/tool-reference.md) | [Changelog](./CHANGELOG.md) | [Contributing](./CONTRIBUTING.md) | [Troubleshooting](./docs/troubleshooting.md) | [Design Principles](./docs/design-principles.md)
 
 ## Key features
 

docs/design-principles.md

@@ -0,0 +1,10 @@
+# Design Principles
+
+These are rough guidelines to follow when shipping features for the MCP server.
+Apply them with nuance.
+
+- **Agent-Agnostic API**: Use standards like MCP. Don't lock in to one LLM. Interoperability is key.
+- **Token-Optimized**: Return semantic summaries. "LCP was 3.2s" is better than 50k lines of JSON. Files are the right location for large amounts of data.
+- **Small, Deterministic Blocks**: Give agents composable tools (Click, Screenshot), not magic buttons.
+- **Self-Healing Errors**: Return actionable errors that include context and potential fixes.
+- **Human-Agent Collaboration**: Output must be readable by machines (structured) AND humans (summaries).