Newsletter: allow HTML in Newsletter Welcome message

[holdercp]

Impacted plugin

Jetpack

What

Allow HTML in Newsletter Welcome email.

Currently, we strip out all HTML other than <a/> tags from the welcome email.

The documentation for the Welcome message suggests you are able to use HTML in the message, which is causing confusion for users. See 9604-gh-Automattic/jpop-issues

The current sanitation approach was carried over from other subscription option fields and was not intentionally applied to the welcome message.

There are already issues open to improve the editing UX for the message because currently we only provide a simple textarea for editing the message. In the meantime however, we could still lighten the sanitation approach so users could use HTML if they choose to.

How

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] User Interaction & Engagement: The issue is about enhancing the newsletter experience, which is directly related to user engagement.
• [Feature] Jetpack: The issue specifically mentions the Jetpack plugin, indicating that the change relates to its functionality.
• [Feature] Subscriptions: The enhancement focuses on the newsletter's welcome message, which is part of the subscription process.
• [Feature] Wpcom Newsletter Widget: This issue targets an improvement in the welcome email of newsletters, relevant to widgets that manage newsletter functionality.

[holdercp]

@StefMattana ping.

[davemart-in]

This is more of a feature request than a bug. Removing from the bugs board.

[simison]

There's a bit more HTML now allowed with change from January, but I'd recommend looking directly at just pulling the content from custom post type or template, edited in Gutenberg:

• Allow customizing welcome email in Gutenberg #36581

It should not be very complicated technically and make the welcome posts match regular posts, with email previews available and all that.