×× or × (same thing) U+00D7 multiplication sign
× same character with a strong font weight
⨯⨯ U+2A2F Gibbs product
✖✖ U+2716 heavy multiplication sign
There's also an emoji ❌ if you support it. If you don't you just saw a square = ❌
I also made this simple code example on Codepen when I was working with a designer who asked me to show her what it would look like when I asked if I could replace your close button with a coded version rather than an image.
<ul><li class="ele"><div class="x large"><b></b><b></b><b></b><b></b></div><div class="x spin large"><b></b><b></b><b></b><b></b></div><div class="x spin large slow"><b></b><b></b><b></b><b></b></div><div class="x flop large"><b></b><b></b><b></b><b></b></div><div class="x t large"><b></b><b></b><b></b><b></b></div><div class="x shift large"><b></b><b></b><b></b><b></b></div></li><li class="ele"><div class="x medium"><b></b><b></b><b></b><b></b></div><div class="x spin medium"><b></b><b></b><b></b><b></b></div><div class="x spin medium slow"><b></b><b></b><b></b><b></b></div><div class="x flop medium"><b></b><b></b><b></b><b></b></div><div class="x t medium"><b></b><b></b><b></b><b></b></div><div class="x shift medium"><b></b><b></b><b></b><b></b></div></li><li class="ele"><div class="x small"><b></b><b></b><b></b><b></b></div><div class="x spin small"><b></b><b></b><b></b><b></b></div><div class="x spin small slow"><b></b><b></b><b></b><b></b></div><div class="x flop small"><b></b><b></b><b></b><b></b></div><div class="x t small"><b></b><b></b><b></b><b></b></div><div class="x shift small"><b></b><b></b><b></b><b></b></div><div class="x small grow"><b></b><b></b><b></b><b></b></div></li><li class="ele"><div class="x switch"><b></b><b></b><b></b><b></b></div></li></ul>