This is managed by one of the filters of the HtmlEditor (Rad Editor), the ConvertToXhtml filter to be more specific.
In order to only disable this filter and leave the rest in place you can navigate to
Administration -> Settings -> Advanced -> Appearance
and find the following fields:
Rad Editor's content filters
RadEditor filters for Content block widget
Then please enter the following filters in the above fields:
RemoveScripts, EncodeScripts, StripCssExpressions, StripDomEventAttributes, FixUlBoldItalic, IECleanAnchors, FixEnclosingP, ConvertTags, ConvertFontToSpan, OptimizeSpans, IndentHTMLContent, ConvertCharactersToEntities, PdfExportFilter, ConvertInlineStylesToAttributes
The above filters include all the default filters except the ConvertToXhtml.
You can also turn them off completely, please observe the following KB-article;
Also as a reference, you can re-write your own filter should you desire,
more on the Custom Content Filters:
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