Cannot write CSS in the "Style" property of the XWiki default skin

Steps to reproduce:

• edit the XWiki default skin inline mode (the base skin must be set to "flamingo")
• copy the following CSS table rules in the "Style" field

th, td {
 border: 1px solid black; 
 text-align: center;
}

th {
 background: rgb(149, 179, 215);
}

• after saving the page notice how the display is broken (see the attached screenshot)

Should we not write CSS in the skin but LESS? Could we have a solution like the one implemented for "XWiki.StyleSheetExtensions" objects i.e. choose whether the content is LESS or CSS?