Custom block created with GUI stripping some  tags, not all of them I am creating a custom block in D8; the block includes the following HTML:

&lt;div class="header__action-items"&gt;
  &lt;a class="btn btn-large" href="help"&gt;Donate Now&lt;/a&gt;
  &lt;a class="btn btn-large" href="me"&gt;Connect With Us&lt;/a&gt;
  &lt;a class="nav-toggle nav-toggle-menu icon-menu" href="#nav"&gt;Menu&lt;/a&gt;
&lt;/div&gt;


I have saved the block using the Full HTML format and placed it into the header region, here is a screenshot of what is added to the page:



I have disabled JS and re-loaded the page to make sure nothing else was happening.  Here is a screenshot of the edit screen:
There was a similar question recently and the problem was CKEditor, could be the same here.

If you want to be sure that your markup is not altered in any way, create a text format without ckeditor and use that.