/* SEO optimized CSS - minimal, fast, responsive (CSS体积小加载快有助于Bing蜘蛛评分) */
:root {
    --primary-color: #E2231A; /* Youdao red */
    --text-color: #333;
    --bg-color: #f9f9f9;
}
* { margin: 0; padding: 0; box-sizing: border-box; }
body { font-family: 'PingFang SC', 'Microsoft YaHei', sans-serif; line-height: 1.6; color: var(--text-color); background: var(--bg-color); }
.container { width: 90%; max-width: 1200px; margin: 0 auto; padding: 0 15px; }
a { text-decoration: none; color: inherit; }

/* Header */
.site-header { background: #fff; box-shadow: 0 2px 5px rgba(0,0,0,0.1); position: sticky; top: 0; z-index: 100; }
.site-header .container { display: flex; justify-content: space-between; align-items: center; padding: 15px; }
.logo h1 { font-size: 24px; color: var(--primary-color); }
.main-nav ul { display: flex; list-style: none; gap: 20px; }
.main-nav a { font-weight: bold; padding: 5px 10px; border-radius: 4px; transition: 0.3s; }
.main-nav a:hover, .main-nav a.active { color: #fff; background: var(--primary-color); }

/* Hero */
.hero-section { background: linear-gradient(135deg, #fff, #f0f0f0); padding: 80px 0; text-align: center; }
.hero-section h2 { font-size: 36px; margin-bottom: 20px; color: var(--primary-color); }
.hero-section p { font-size: 18px; color: #555; margin-bottom: 30px; }
.btn { display: inline-block; padding: 12px 30px; border-radius: 30px; font-size: 16px; font-weight: bold; transition: 0.3s; }
.btn-primary { background: var(--primary-color); color: #fff; }
.btn-primary:hover { background: #c11c15; }
.btn-secondary { background: #fff; color: var(--primary-color); border: 2px solid var(--primary-color); margin-left: 15px; }
.btn-secondary:hover { background: var(--primary-color); color: #fff; }

/* Features */
.features-summary { padding: 60px 0; background: #fff; }
.features-summary h3 { text-align: center; font-size: 28px; margin-bottom: 40px; }
.feature-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(300px, 1fr)); gap: 30px; }
.feature-card { background: #f9f9f9; padding: 30px; border-radius: 8px; text-align: center; box-shadow: 0 4px 6px rgba(0,0,0,0.05); }
.feature-card h4 { font-size: 20px; margin-bottom: 15px; color: var(--primary-color); }

/* Download Section */
.download-section { padding: 60px 0; }
.download-section h3 { font-size: 28px; margin-bottom: 20px; }
.download-list { list-style: none; margin: 30px 0; }
.download-list li { background: #fff; padding: 20px; margin-bottom: 15px; border-radius: 8px; display: flex; justify-content: space-between; align-items: center; box-shadow: 0 2px 4px rgba(0,0,0,0.05); }
.btn-download { background: #4CAF50; color: #fff; padding: 8px 20px; border-radius: 4px; }
.btn-download:hover { background: #45a049; }

/* Content Section (Inner pages) */
.content-section { padding: 50px 0; background: #fff; min-height: 50vh;}
.content-section h2 { font-size: 30px; margin-bottom: 20px; color: var(--primary-color); border-bottom: 2px solid #eee; padding-bottom: 10px; }
.content-section h3 { font-size: 22px; margin: 25px 0 15px 0; }
.content-section p { margin-bottom: 15px; font-size: 16px; color: #444; }

.seo-text { font-size: 14px; color: #777; margin-top: 30px; padding: 15px; background: #fff; border-left: 4px solid var(--primary-color); }

/* Footer */
.site-footer { background: #333; color: #ccc; text-align: center; padding: 30px 0; margin-top: 40px; }
.site-footer p { margin-bottom: 10px; }
.site-footer strong { color: #fff; font-weight: normal; }

/* Responsive */
@media (max-width: 768px) {
    .site-header .container { flex-direction: column; gap: 15px; }
    .hero-section h2 { font-size: 28px; }
    .download-list li { flex-direction: column; gap: 15px; text-align: center; }
}