Improve HTML reprs (#10816)

* Less vertical whitespace in HTML reprs

* ensure consistent line-height in google colab

* Refactor DataTree HTML repr

* Collapsable DataTree nodes

* more formatting

* Tweaks

* Add tests and truncation for many child nodes

* add missing DataTree.xindexes into repr

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Add whats-new entry for DataTree HTML repr improvements

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* Fix ruff PERF401: use list.extend instead of append in loop

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* Adjust style per review

* Use browser default focus outline style

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Author: 3 people

doc/whats-new.rst

@@ -14,6 +14,11 @@ v2025.12.1 (unreleased)
 New Features
 ~~~~~~~~~~~~
 
+- Improved :py:class:`DataTree` HTML representation: groups are now collapsible
+  with item counts shown in labels, large trees are automatically truncated
+  using ``display_max_children`` and ``display_max_html_elements`` options,
+  and the Indexes section is now displayed (matching the text repr) (:pull:`10816`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
 - :py:meth:`Dataset.set_xindex` and :py:meth:`DataArray.set_xindex`
   automatically replace any existing index being set instead of erroring
   or needing needing to call :py:meth:`drop_indexes` first (:pull:`11008`).