line_50_percent.html: margin-top: -4.5px in ref seems unnecessary #6
Description
In /webvtt/rendering/cues-with-video/processing-model/line_50_percent-ref.html, the <span class="cue"> is given margin-top: -4.5px. I believe this is put there to render the cue exactly vertically in the middle of the video. However, I do not think this is the correct position. According to section 3.3 of the WebVTT spec under "A line alignment", by default the line alignment is set to start (which is the top edge for horizontal cues). Based on this, I do not think the cue is supposed to be exactly centered, but rather the top edge should be positioned at the center.
To fix this, we can simply remove margin-top: -4.5px from line_50_percent-ref.html.