I don't like this. This could be implemented as a JS library. I believe browsers should provide the minimal API so that they are smaller and easier to create. As for safe alternative to innerHTML, it is called innerText.
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Libraries can surely do the same job, but then the exact behavior would vary among a sea of those libs. Having specs defined [0] for such an interface would hopefully iron out much of these variations, as well as enabling some performance gains.
[0]: https://wicg.github.io/sanitizer-api/#dom-element-sethtml