discourse/plugins/discourse-details/assets/javascripts/lib/discourse-markdown/details.js.es6

const rule = {
  tag: 'details',
  before: function(state, tagInfo) {
    const attrs = tagInfo.attrs;
    state.push('bbcode_open', 'details', 1);
    state.push('bbcode_open', 'summary', 1);

    let token = state.push('text', '', 0);
    token.content = attrs['_default'] || '';

    state.push('bbcode_close', 'summary', -1);
  },

  after: function(state) {
    state.push('bbcode_close', 'details', -1);
  }
};

export function setup(helper) {
  helper.whiteList([
    'summary',
    'summary[title]',
    'details',
    'details[open]',
    'details.elided'
  ]);

  helper.registerPlugin(md => {
    md.block.bbcode.ruler.push('details', rule);
  });
}