What does the letter w stand for generally in young people's writings, like in
:

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帰宅途中に突然変なノリが始まったww

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or in :

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それが好きってww﻿

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I found them on Youtube
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2017-08-10T08:53:05.957
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2017-08-10T08:56:57.583
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What does w stands for in young people's writings?