TYPO3 Core

Some errors / warnings are handled in PageLayoutController

These use a " callout " message type with the InfoBox template:

<f:be.infobox title="{title}" state="{state}">
<f:format.raw>{message}</f:format.raw>
</f:be.infobox>

which results for example in:

<div class="callout callout-danger"><div class="media"><div class="media-left"><span class="fa-stack fa-lg callout-icon"><i class="fa fa-circle fa-stack-2x"></i><i class="fa fa-times fa-stack-1x"></i></span></div><div class="media-body"><h4 class="callout-title">shortcut with missing target</h4><div class="callout-body">
    Page is configured to be a link but there is actually no valid target defined.
</div></div></div></div>

Other messages (for example "Unused elements detected on this page") use a flashmessage styling:

<div class="typo3-messages"><div class="alert alert-warning">  <div class="media">    <div class="media-left">      <span class="fa-stack fa-lg">        <i class="fa fa-circle fa-stack-2x"></i>        <i class="fa fa-exclamation fa-stack-1x"></i>      </span>    </div>    <div class="media-body">      <h4 class="alert-title">Unused elements detected on this page</h4>      <p class="alert-message">These elements don't belong to any of the available columns of this page. You should either delete them or move them to existing columns. We highlighted the problematic records for you.</p>    </div>  </div></div></div>

IMHO, the same styling (for callouts) should be used in these cases as well.