I have difficulty in removing border from a specific PrimeFaces .
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As mentioned by BalusC, the border is set by PrimeFaces on the generated tr and td elements, not on the table. However when trying with PrimeFaces version 5, it looks like there is a more specific match from the PrimeFaces CSS .ui-panelgrid .ui-panelgrid-cell > solid which still result in black borders being shown when appyling the style suggested.
Try using following style in order to overide the Primefaces one without using the !important declaration:
.companyHeaderGrid tr, .companyHeaderGrid td.ui-panelgrid-cell {
border: none;
}
As mention make sure your CSS is loaded after the PrimeFaces one.