According to Wikipedia, HTML 4 was published in 1997, XML in 1998 and XHTML 1.0 in 2000, so it is unlikely there was any backporting from XHTML 1.0 to HTML 4.

According to Wikipedia, HTML 4 was published in 1997, XML in 1998 and XHTML 1.0 in 2000, so it is unlikely there was any backporting from XHTML 1.0 to HTML 4. The issue around DTD's is confusing because browsers use the <!DOCTYPE>-tag as a signal to switch between quirks and standards rendering modes. This might suggest a connection between the DTD's and the rendering modes, but in reality browsers have always ignored the DTDs. They just match the <!DOCTYPE> against some predefined strings to determine the mode. And the DTD's was never designed for this purpose, it was just a hack introduced by browser vendors. > Rendering changes were one of the reasons few developers opted into XHTML 1.x Strict. XHTML 1.x strict was rendered the same way as the corresponding HTML in standards mode. But given the confusion around the issues, it is possible some developers thought it would render differently.