两件事：
1.您看不到颜色变化，因为td在tr前面，从而使背景变得模糊
1.内联样式具有更高的特异性，并且将始终覆盖样式（因此不建议使用它们）
修复：

table tr:hover td {
  background-color: #f5f5f5 !important;
}

添加td可修复遮挡问题
添加!important可修复内联样式问题

const htmlContent = `
    <style>
        table tr:hover td {
        background-color: #f5f5f5 !important;
    }
    </style>
    
    <header style="padding: 20px; text-align: center;position: relative">
        <div class="contact-info" style="position: absolute; margin-top: 21px; margin-right: 25px; top: 0; right: 0; font-family: Arial, sans-serif; font-size: 13px; color: #fff; text-align: right;">
            Contact: +91-8491064038, +91-9622259639<br>
            Email: info@azeestravels.com
        </div>
        <div style ="padding: 20px;text-align: center;background-color: #333;">
            <h1 style="display: inline-block;font-family: Arial, sans-serif; font-size: 48px; font-weight: bold; margin: 0; color: #333; text-shadow: 2px 2px #ccc; background-image: linear-gradient(to bottom, #ffcc00, #ff6600); -webkit-background-clip: text; -webkit-text-fill-color: transparent;">Azees Travel</h1>
            <p id="tag" style="font-family: Arial, sans-serif; font-size: 16px; color: white; font-weight: bold;">Dream it, visit it</p></div>
        <div>
        <p id = "greet" style="font-family: Georgia, serif; font-size: 16px; line-height: 2.5; max-width: 1232px; padding: 30px; background-color: #fff; color: #333; box-shadow: 0px 0px 10px rgba(0, 0, 0, 0.5); margin: 0; margin-top: 10px;"><span id="gretD" style="font-size:29px">
            Greetings from Azees Travel!</span><br>
            Great to hear you are interested in visiting Kashmir.You can take a look at our scheduled upcoming tour for you. All of our Kashmir Tours include visits to Pahalgam, Gulmarg, 
            Sonamarg and Srinagar.It also includes accommodation and transfers, when you arrive from airport and depart to Airport.We use quality accommodation throughout the tour and make 
            use of new air-conditioned 4-wheelers. You are in good hands with our certified speaking guides all of whom work for us full time.We guarantee every tour and are committed to 
            making sure you have a great experience. We wish you a very happy journey and we would love to make your trip more comfortable and enjoyable.Team AZEES TRAVEL wishes you a great 
            journey & we would love to hear from you upon final package confirmation.
            Thank you .....!</p>
        </div>
        <h3 style="text-align:left;color: red; text-decoration: underline;">Summary:</h3>
        <table style="border-collapse: collapse; width: 100%; margin-bottom: 20px; background-color: #f8f8f8; font-size: 16px; color: #333;">
            <tr>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;"><Tour Duration:</td>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;">5 Days</td>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;">Adults:</td>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;">8</td>
            </tr>
            <tr>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;">Tour Start Date:</td>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;">April 1, 2023</td>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;">Kids:</td>
                <td style="padding: 10px; text-align: left; border-bottom: 1px solid #ddd; background-color: #e6e6e6; font-weight: bold;">8</td>
            </tr>
        </table>
        <h3 style="text-align:left;color: red; text-decoration: underline;">The Travel Itinerary:</h3>
    </header>

    </form>`;

document.body.innerHTML = htmlContent

正如注解所暗示的，td丢失了。但是还要在css中添加！important，否则它不会覆盖表格中使用的背景色

<style>
        table tr:hover td {
        background-color: #f5f5f5 !important;
    }
    </style>