When Open Sans was in use, the `300`, `400`, and `600` weights were loaded. `400` is the equivalent of `normal`; however, `bold` is equivalent to `700`, not `600`. With the move to system fonts, we need to be specific rather than relying on the lack of a `700` weight. Not all system fonts include a `600` weight; in those instances, they will use the `bold`/`700` weight.
The WordPress CSS Coding Standards have been updated accordingly.
props coderste.
see #36753.
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