The style guide's HTML file does not itself follow it's own rules

From the guide

> #Omit optional tags (optional).
>
> For file size optimization and scannability purposes, consider omitting optional tags. The HTML5 specification defines what tags can be omitted.
>
> (This approach may require a grace period to be established as a wider guideline as it’s significantly different from what web developers are typically taught. For consistency and simplicity reasons it’s best served omitting all optional tags, not just a selection.)
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> Spending money, spending bytes
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>
>

Sic.

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> Saving money, saving bytes
>

Qed.

then compare to the style guide's own HTML

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Google HTML/CSS Style Guide