[RavSinghChandan]

e.g. Fixes #0

Checklist:

• A unit test is covering the code added / modified by this PR

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• A mention of the change is present in CHANGELOG.md

• This PR is ready to be merged

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[RavSinghChandan]

bgcolor="eeeeee" or <font color="ff0000"> raised ValueError: ff0000 does not start with #. Browsers read six hex digits without the leading # as that hex color (HTML's legacy color parsing), and older HTML, such as many email templates, writes colors this way. color_as_decimal() now adds the # for exactly six hex digits. Shorter forms like f00 are not, since browsers read those differently, and still raise as before. Added tests.