pandas to_html using the .style options or custom CSS?

It's not an extravagant / pythonic solution. I inserted the link to a direct css file before the html code made by the to_html () method, then I saved the whole string as an html file. This worked well for me.

dphtml = r'<link rel="stylesheet" type="text/css" media="screen" href="css-table.css" />' + '\n'
dphtml += dp.to_html()

with open('datatable.html','w') as f:
    f.write(dphtml)

Once you add style to your chained assignments you are operating on a Styler object. That object has a render method to get the html as a string. So in your example, you could do something like this:

html = (
    df.style
    .format(percent)
    .applymap(color_negative_red, subset=['col1', 'col2'])
    .set_properties(**{'font-size': '9pt', 'font-family': 'Calibri'})
    .bar(subset=['col4', 'col5'], color='lightblue')
    .render()
)

Then include the html in your email instead of a df.to_html().