Redmine

Raw <style> tags are deliberately excluded in Redmine to avoid potential issues where arbitrary markdown text could alter the user interface of the surrounding page.

If desired, you can use the style attribute on most HTML tags instead where we allow a limited set of CSS rules to be used, e.g.:

<h1 style="color: red">This is a header</h1>
<p style="color: blue;"> This is a paragraph</p>

If you want to permanently alter the rendering of tags within your own Redmine installation, you can also install your own theme where you can use arbitrary CSS rules. See HowTo create a custom Redmine theme for details. To change the rendered markdown text in your custom theme, note that it will always be enclosed in a tag (usually a div or p) with the wiki class. Your custom Redmine theme could thus define something like:

.wiki h1 {color:red;}
.wiki p {color:blue;}