Docs: Strawman design principles

Review URL: https://codereview.chromium.org/676093002

specs/design.md

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+Design Principles
+=================
+
+* There should be no objects that represent live state that reflects
+  some other state, since they are expensive to maintain. e.g. no
+  HTMLCollection.
+
+* Property getters should be efficient. If an operation is inefficient
+  it should be a method instead. e.g. document.getForms(), not
+  document.forms.
+
+* There should be no APIs that require synchronously computing layout
+  (or other expensive operations).
+
+* Any API that can be implemented in terms of another is a convenience
+  API and should be implemented in a framework, not as part of the
+  core. e.g., no document.forms.
+
+ - having APIs for performance reasons is fine (e.g. querySelector()
+   could be implemented by crawling but it would be so much faster if
+   it could use the runtime's ID hashtables that it's ok to support
+   natively)
+
+* APIs that encourage bad practices should not exist. e.g., no
+  document.write(), innerHTML, insertAdjacentHTML(), etc.
+
+* If we expose some aspect of a mojo service (e.g. touch events) we
+  should expose/wrap all of it (e.g. mousewheel) so that there's no
+  cognitive cliff when interacting with that service
+
+* APIs should always spell acronyms like words (findId, not findID;
+  XmlHttpRequest, not XMLHttpRequest)