There are many email clients and none work like others. The lib must rewrite the HTML content to make it work everywhere. The list of what should be done :
some clients doesn't handle shorthand CSS properties. The lib must rewrite the CSS property to expand them (padding: 4px; -> padding: 4px 4px 4px 4px;)
add border=0 on images and tables to avoid ugly borders on some clients
some clients doesn't handle well padding. The lib could add a parent element and use margins instead of padding
some CSS properties are not managed at all. The lib must convert it to old HTML properties:
handle cellpadding/cellspacing
handle color/font
handle img[alt]
handle align/valign
handle bgcolor/color
...
convert the HTML into XHTML version
remove HTML comments (except conditional comments for Outlook)
transform CSS3 properties/layouts into basic HTML (flex into tables, gradients to images...)
clean the HTML (remove attributes added for soruce code useful to the lib but that should not be visible at the end)
Every transformation is required by at least one email client. The user should be able to select which transformations to apply or even better which email clients to target. So disabling a client should also disabling all replacements that are only required by this client. Disabling several clients should disabling replacements that are no more useful for tagetted clients.
There are many email clients and none work like others. The lib must rewrite the HTML content to make it work everywhere. The list of what should be done :
Every transformation is required by at least one email client. The user should be able to select which transformations to apply or even better which email clients to target. So disabling a client should also disabling all replacements that are only required by this client. Disabling several clients should disabling replacements that are no more useful for tagetted clients.
See https://www.campaignmonitor.com/css/ for support of features
See these links for transformations needed: http://www.alsacreations.com/actu/lire/502-e-mail-et-css-du-nouveau.html http://www.pompage.net/traduction/cssemail http://www.pompage.net/traduction/emails-html-dompter-la-bete https://www.campaignmonitor.com/blog/post/3363/updated-applying-a-background-image-to-html-email