When you create your email template and add your html code to email newsletters, you can only use allowed tags.

List of allowed html-tags and their attributes:

a: href, title, name, style, id, class, shape, coords, alt, target.
b: style, id, class.
br: style, id, class.
big: style, id, class.
blockquote: title, style, id, class.
caption: style, id, class.
code: style, id, class.
del: title, style, id, class.
div: title, style, id, class, align.
dt: style, id, class.
dd: style, id, class.
font: color, size, face, style, id, class.
h1: style, id, class, align.
h2: style, id, class, align.
h3: style, id, class, align.
h4: style, id, class, align.
h5: style, id, class, align.
h6: style, id, class, align.
hr: style, id, class.
i: style, id, class.
img: style, id, class, src, alt, height, width, title.
ins: title, style, id, class.
li: style, id, class.
map: shape, coords, href, alt, title, style, id, class, name.
ol: style, id, class.
p: style, id, class, align.
pre: style, id, class.
s: style, id, class.
small: style, id, class.
strong: style, id, class.
span: title, style, id, class, align.
sub: style, id, class.
sup: style, id, class.
table: border, width, style, id, class, cellspacing, cellpadding.
tbody: align, valign, style, id, class.
td: width, height, style, id, class, align, valign, colspan, rowspan.
tfoot: align, valign, style, id, class, align, valign.
th: width, height, style, id, class, colspan, rowspan.
thead: align, valign, style, id, class.
tr: align, valign, style, id, class.
u: style, id, class.
ul: style, id, class.
php: id.
html: xmlns.
head
body
meta: content, name, http-equiv.
title
style: Editor::STYLIST_TAG_ATTR, type.
link: type, rel, href.