When styling an xf:message, certain CSS properties have no effect

When using CSS classes, xf\:message, pe-message-header, .pe-header-text and .pe-message-footer to style all types of xf:messages, some properties, when set are not applied to the message.

Specifically:

Setting 'width' value for .pe-message-header has no effect
Setting 'padding' value for .pe-message-header has no effect
Setting 'margin' value for pe-header-text has no effect

See the attached test form and the comments inline.