For public-facing HTML content provided by the user,
`<style>` tags and `style` attributes should be stripped
automatically, as they can easily be abused to deface
pages for abusive users and spammers.
<style> tags were already stripped, the optional `strip_style`
for fields.html enables the automatic stripping of style
attributes.
This is opt-in because custom style attributes are still
desirable in trusted HTML fields.
Olivier Dony
13476c844d