Parse and append an HTML or SVG string. I use it a lot for appending a template, instead of generating it with d3.
d3.select('.container').appendHTML('<div><svg><g><rect width="50" height="50" /></g></svg></div>');Unlike using .html, .appendHTML can append multiple elements
d3.select('.container').html('<span id="a"></span>');
d3.select('.container').html('<span id="b"></span>'); // will replace content
d3.select('.container').appendHTML('<span id="c"></span>'); // will append content