Sphinx version mismatch between local and Read the Docs builds causes inconsistent output

Goal

Produce identical HTML output when building a Sphinx project locally and on Read the Docs, despite the platform pinning a specific Sphinx version in its build image.

Constraints and uncertainty

Read the Docs builds run in a controlled Docker image that includes a fixed Sphinx release (for example, 7.2.x in the current "ubuntu-22.04" image). Local environments often have a newer or older Sphinx version, and the project's requirements.txt or .readthedocs.yaml may not explicitly constrain the version. Extensions such as sphinx-copybutton or myst-parser can also behave differently across Sphinx minor releases, leading to missing CSS classes, altered HTML structure, or failed directives that only appear in the hosted build logs.

Questions

1. Which configuration keys in .readthedocs.yaml reliably pin the Sphinx version used by the platform without breaking the automatic dependency installation?
2. How can a contributor verify locally—without pushing—that the exact same Sphinx version and extension set will be used on Read the Docs?
3. Are there documented cases where a Sphinx point release introduced a breaking change that Read the Docs has not yet mitigated in its build image?