No no. The thickness of the underline is browser dependent and cannot be affected in CSS (or HTML).
In CSS3 Text, the text-underline-width property has been proposed. But this was not enough interest, and he was excluded from the project in 2005. It was probably never implemented.
The usual workaround is to use a lower border instead of underlining. However, note that this is a completely different matter. The lower border is below the line of the line, while the underline is usually found in the source text and, therefore, shortens the letter descriptors. As a rule, the lower bound is better for readability than underlining, but it differs from the typographic tradition.
The following example demonstrates the differences.
<span style="text-decoration: underline">jgq</span> <span style="border-bottom: solid 1px">jgq</span> <span style="border-bottom: solid 4px">jgq</span>
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