How can I make a simple line across the screen, whether or not text is present. Formatting nbsp's is sloppy and silly. Trying to do it with box borders, but I just can't seem to get the code right.

Try this (horizontal ruler in HTML):

<hr/>

Omg that is fabulous. Is there a way to change it's color?

I figured it out, using hr style, however there is a weird gap underneath inside my game,

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sample text.

like there is an invisible line break above and below.

Alright, this is an odd work around but here is what I came up with incase it'll help someone else someday.<hr style="margin-bottom:-15px">
changing the amount of pixels to suit. My in game text size is set to 15,

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so -15 looks like a normal strike out above the text.

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-25 like an overline,

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-45 line an underline.
and so forth.

To also affect thickness and color, this needs to be added:
<hr style="border:.5px solid red; margin-bottom:12px">:

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.5px is the smallest possible. Hex#s don't seem to work here, and I've only had success using color labels. For whatever reason, it also won't work without the word solid.