As the HTML code had a tendency of causing unintended issues, we'd blocked the the use of HTML code within PMs. However, understanding that users rely heavily on sending images and url links in PMs, those tags were whitelisted, hence they're now working.
But thanks for letting us know your need for other html tags, such as backgrounds, text alignment, italics. The team will be looking into whitelisting them as well. If there are more html tags you'd like to use, please let us know, and we'll work to accommodate these as well.