html snippet in LTR directionality regardless of overall
context. If the html piece was enclosed by a tag, the direction will be
applied to existing tag, otherwise a span tag will be added as wrapper.
For this reason, if html snippet start with with tag, this tag must
enclose the whole piece. If the tag already has a direction specified,
this new one will override existing one in behavior (tested on FF and IE).
static String enforceLtrInHtml(String html) => _enforceInHtmlHelper(html, 'ltr');