kramdown-markdown-it.js

const md = require('markdown-it')();
const markdownItAttrs = require('markdown-it-attrs');

const src = `
# header {: .title-class #title-id }
some text {: with=attrs and="attrs with space" }
more body copy with some *emphasis*{: .emph-class } thrown in {: .para-class}
- item 1
- item 2 {: .item-class }
- item 3
{: .list-class }
`;

// render markdown
md
  .use(
    markdownItAttrs,
    // support kramdown
    { leftDelimiter: '{:' }
  )
  .render(src);