I have created a HTML file:
Set the bottom margin to zero:
And the next time when I open this file an extra bottom margin appear:
Where does this extra margin come from ?
IMO this is a bug, and fdo#62445 appears to be the related one. If you set Line Spacing to a Fixed value (e.g., 14.1pt), saving and re-opening the file results in not only the Line Spacing being reset to At Least but also this type of code in the HTML:
<STYLE TYPE="text/css">
<!--
@page { margin: 72pt }
P { margin-bottom: 0pt; line-height: 14.1pt; page-break-before: auto }
A:link { so-language: zxx }
-->
</STYLE>
<P STYLE="margin-bottom: 14.2pt">Hun mä éiweg Hémecht, ass Himmel dämpen gehéiert fu. Da geet dann Gesträich dir, muerges Hemecht gin mä. Welt Milliounen da den. Wäit Hémecht Blieder mir dé, fu Hierz Gaart rei.</P>
<P STYLE="margin-bottom: 14.2pt">Dé Gaart Milliounen der. Wat Welt riede erwaacht an. Jo net päift Poufank, sech gefällt ass hu, denkt d'Leit de vun. Ké Duerf bleiwe mat. Wand heemlech schaddreg gei vu, duurch Blieder um och, ke fort Freiesch wee.</P>
Those margin-bottom attributes against each paragraph are part of the problem. Sadly though this is only part of the problem as browsers like Iceweasel/Firefox will add a paragraph margin to basic text. If you edit the above (using a text editor) to read:
<P>Hun mä éiweg Hémecht, ass Himmel dämpen gehéiert fu. Da geet dann Gesträich dir, muerges Hemecht gin mä. Welt Milliounen da den. Wäit Hémecht Blieder mir dé, fu Hierz Gaart rei.</P>
<P>Dé Gaart Milliounen der. Wat Welt riede erwaacht an. Jo net päift Poufank, sech gefällt ass hu, denkt d'Leit de vun. Ké Duerf bleiwe mat. Wand heemlech schaddreg gei vu, duurch Blieder um och, ke fort Freiesch wee.</P>
You will still get an implied (by Iceweasel/Firefox) top paragraph margin e.g.,
It is only through editing the CSS definition to explicitly set the top margin to 0pt that the desired display is possible:
P { margin-bottom: 0pt; margin-top: 0pt; line-height: 14.1pt; page-break-before: auto }
As soon as you re-edit the HTML via Writer though this is overwritten. I think the “HTML (StarWriter)” filter needs to be more explicit in its setting of these margin values, based on the options and values of the Indents & Spacings tab (as you indicate).