In Sigil I have set up a 'clip' to place the surrounding <div>s...so all I have to do is highlight whatever section I want vertically centered...then click on the clip. For more info, take a look in the "Clips" section of the
Sigil Users Guide.
Code:
Clip:
Name: Text:
V-Ctr <div class="v-ctr">\n<div>\n\1\n</div>\n</div>