The Rich Text Widget behaves in some unobvious ways i.e. it allows some style tags, but strips out others - the same for some features of the text widget such as the container. Please could someone comprehensively check this info and add to the help about the text Widget. Note, as a start, Jan Dauz kindly checked out the features and supplied the below information as a starter list (which could immediately be added to the help section) - many thanks Jan!...  and kudos also to Alenka Prezelj for pointing me to post my suggestion here.

Supported in Text widget

background-color, color, display, float, font-size, margin-left, margin-right, text-align, text-decoration

Unsupported in Text widget

animation, background-image, border, border-bottom, border-top, border-left, border-right, border-radius, box-shadow, direction, font-family, flex, font-weight, font-style, width, height, top, right, left, line-height, letter-spacing, margin, margin-top, margin-bottom, padding, padding-left, padding-right, padding-top, padding-bottom, opacity, outline, overflow, position, pointer-events, text-shadow, text-transform, transform, transition, visibility, white-space, word-spacing, z-index