Thank you for the report.
Would you have a html+JS reproduction of the problem?
In the trunk code, he intention is that this is handled at ANGLE level, at:
  bool ValidMipLevel(const Context *context, TextureType type, GLint level)

Yes, I run below codes:
PASS: Bad target (in assertFail)(function(){ 
  gl.copyTexImage2D(gl.FLOAT, 0, gl.RGBA, 0,0, 16,16,0); 
})
PASS: Bad internal format (in assertFail)(function(){ 
  gl.copyTexImage2D(gl.TEXTURE_2D, 0, gl.FLOAT, 0,0, 16,16,0); 
})
Fail: Negative level (in assertFail)(function(){ 
  gl.copyTexImage2D(gl.TEXTURE_2D, -1, gl.RGBA, 0,0, 16,16,0); 
})
The last one causes the crash.
I don't know ValidMipLevel until you told me actually.

If you have a standalone runnable .html file to try, I can try it.
Additionally, if possible a  backtrace of the crash is always helpful.
Also useful information would be to know if the version of WebKit you are using is close to current trunk or older?

Try this page:

https://registry.khronos.org/webgl/conformance-suites/1.0.3/conformance/more/functions/copyTexImage2DBadArgs.html

That page passes for current trunk at least on Mac.
We've recently, within last 6 months, removed legacy WebGL backend implementation which might not have the validation. 

In case you are testing an older WPE build, it might be triggering the old code path. A backtrace of the crash would help clarify that.

We are really using the older version webkit, but I also checked the latest version webkit it has the same codes which I pasted below.

void WebGLRenderingContextBase::copyTexImage2D(GCGLenum target, GCGLint level, GCGLenum internalFormat, GCGLint x, GCGLint y, GCGLsizei width, GCGLsizei height, GCGLint border)
{
...
// FIXME: if the framebuffer is not complete, none of the below should be executed.
    tex->setLevelInfo(target, level, internalFormat, width, height, GraphicsContextGL::UNSIGNED_BYTE);
...
}
void WebGLTexture::setLevelInfo(GCGLenum target, GCGLint level, GCGLenum internalFormat, GCGLsizei width, GCGLsizei height, GCGLenum type)
{
...
    m_info[index][level].setInfo(internalFormat, width, height, type);
...
}
It possibly uses minus level as the array index then cause the crash.
Hopefully it is helpful to you!

I just patched below codes in bool WebGLRenderingContextBase::validateTexFuncParameters

  6          synthesizeGLError(GraphicsContextGL::INVALID_VALUE, functionName, "width or height <     0");
  7          return false;
  8      }
  9 +
 10 +    if (level < 0) {
 11 +        synthesizeGLError(GraphicsContextGL::INVALID_VALUE, functionName, "level < 0");
 12 +        return false;
 13 +    }

> I also checked the latest version webkit it has the same codes which I pasted below.
> void WebGLTexture::setLevelInfo(GCGLenum target, GCGLint level, GCGLenum internalFormat, GCGLsizei width, GCGLsizei height, GCGLenum type)

Thanks!
The code has been removed from WebKit around 3 months ago.

https://github.com/WebKit/WebKit/blob/main/Source/WebCore/html/canvas/WebGLTexture.cpp