css3 如何实现圆边框的渐变? 如图

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				<span class="k">left</span> <span class="k">top</span><span class="o">,</span>
				<span class="k">right</span> <span class="k">top</span><span class="o">,</span>
				<span class="k">color</span><span class="o">-</span><span class="n">stop</span><span class="p">(</span><span class="m">0</span><span class="o">,</span><span class="m">#077df8</span><span class="p">)</span><span class="o">,</span>
				<span class="k">color</span><span class="o">-</span><span class="n">stop</span><span class="p">(</span><span class="m">1</span><span class="o">,</span><span class="m">#74baff</span><span class="p">)</span>
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			<span class="k">border-bottom</span><span class="o">-</span><span class="k">left</span><span class="o">-</span><span class="n">radius</span><span class="o">:</span><span class="m">50px</span><span class="p">;</span>
			<span class="k">border-bottom</span><span class="o">-</span><span class="k">right</span><span class="o">-</span><span class="n">radius</span><span class="o">:</span><span class="m">50px</span><span class="p">;</span>
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				<span class="k">color</span><span class="o">-</span><span class="n">stop</span><span class="p">(</span><span class="m">0</span><span class="o">,</span><span class="m">#fff</span><span class="p">)</span><span class="o">,</span>
				<span class="k">color</span><span class="o">-</span><span class="n">stop</span><span class="p">(</span><span class="m">1</span><span class="o">,</span><span class="m">#fff</span><span class="p">)</span>
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				<span class="n">linear</span><span class="o">,</span>
				<span class="k">left</span> <span class="k">top</span><span class="o">,</span>
				<span class="k">right</span> <span class="k">top</span><span class="o">,</span>
				<span class="k">color</span><span class="o">-</span><span class="n">stop</span><span class="p">(</span><span class="m">0</span><span class="o">,</span><span class="m">#fff</span><span class="p">)</span><span class="o">,</span>
				<span class="k">color</span><span class="o">-</span><span class="n">stop</span><span class="p">(</span><span class="m">1</span><span class="o">,</span><span class="m">#74baff</span><span class="p">)</span>
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