Normalize known self-closing HTML elements

[yhatt]

For normalize known self-closing HTML elements, we will apply multi-pass filtering to HTML renderer. When markdown-it's xhtmlOut option is true, the whitelisted HTML elements would be normalized.

Motivation

I'm trying to enhance Marp's collaboration with well-known JavaScript framework: React, Vue, and so on.

I have already tried using Incremental DOM via my markdown-it-incremental-dom plugin, and it works well at Marp Web built on Preact. However, these are not very well known although great modules. To spread Marp ecosystem widely, I'm thinking that we should provide a better experience into frequently used JS frameworks in general.

So I have tried creating React component at my CodeSandbox: https://codesandbox.io/s/v6v4rzx4jl. It has succeeded to render slides with minimum diff by converting rendered HTML into virtual DOM (using htm, he, and style-to-object).

The problem was caused by the whitelisted <br> HTML element. htm parser requires the strict syntax to self-closing element, so user-written <br> will raise error. Thus, we have to normalize self-closing element.