EPPlus ExcelPackage to Html table

ExcelPackageExtension.cs

public static class ExcelPackageExtension
    {
        internal static byte[] HtmlBytes(this ExcelPackage package)
        {
            string html = string.Empty;
            string headerHtml = string.Empty;
            string footerHtml = string.Empty;
            string theadHtml = string.Empty;

            const string header = "{{header}}";
            const string footer = "{{footer}}";
            const string thead = "{{thead}}";

            html += header;
            ExcelWorksheets sheets = package.Workbook.Worksheets;
            foreach (ExcelWorksheet sheet in sheets)
            {
                if (sheet.Dimension == null) continue;

                int noOfCol = sheet.Dimension.End.Column;
                int noOfRow = sheet.Dimension.End.Row;

                html += $"<table>{thead}<tbody>";
                bool trOpen = false, theadOpen = false;
                for (int i = 1; i <= noOfRow; i++)
                {
                    string trHtml = string.Empty;
                    for (int j = 1; j <= noOfCol; j++)
                    {
                        int colspan = 1;
                        int rowspan = 1;
                        if (!sheet.Cells[i, j].Merge || (sheet.Cells[i, j].Merge &&
                                                         IsMergeRange(sheet.Cells[i, j].Address)))
                        {
                            if (sheet.Cells[i, j] != null && sheet.Cells[i, j].Value != null && colspan > 1)
                            {
                                if(!trOpen) 
                                    headerHtml += $"<div class='header'><h2>{sheet.Cells[i, j].Value}</h2></div>";
                                else
                                    footerHtml += $"<div class='footer'>{sheet.Cells[i, j].Value}</div>";
                            }
                            
                            if (sheet.Cells[i, j] != null && sheet.Cells[i, j].Value != null && colspan == 1)
                            {
                                trOpen = true;
                                trHtml += !theadOpen ? $"<th rowspan='{rowspan}' colspan='{colspan}'>" : $"<td rowspan='{rowspan}' colspan='{colspan}'>";
                                trHtml += sheet.Cells[i, j].Value;
                                trHtml += !theadOpen ? "</th>" : "</td>";
                            }
                        }

                        bool IsMergeRange(string address)
                        {
                            colspan = 1;
                            rowspan = 1;
                            foreach (string item in sheet.MergedCells)
                            {
                                string[] sub = item.Split(':');
                                if (sub.Length > 0 && sub[0].Equals(address))
                                {
                                    ExcelRange range = sheet.Cells[item];
                                    colspan = range.End.Column - range.Start.Column + 1;
                                    rowspan = range.End.Row - range.Start.Row + 1;
                                    return true;
                                }
                            }

                            return false;
                        }
                    }

                    if (trOpen)
                    {
                        if (!theadOpen)
                        {
                            theadHtml += $"<thead><tr>{trHtml}</tr></thead>";
                            theadOpen = true;
                        }
                        else
                        {
                            html += $"<tr>{trHtml}</tr>";
                        }
                        
                    }
                }
                html += "</tbody></table>";
            }

            html += footer;

            html = html.Replace(thead, theadHtml);
            html = html.Replace(header, headerHtml); 
            html = html.Replace(footer, footerHtml);
            
            return Encoding.ASCII.GetBytes(html);
        }
    }